fix get_file_method

root@vagrant:~/test# docker run -it --rm -p 10809:10809 -v $(pwd):/opt/nbd -e RUST_BACKTRACE=full dkr.local/nbd-rs:dev --export test1 raw file:/opt/nbd/image1.raw
thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value: Os { code: 2, kind: NotFound, message: "No such file or directory" }', src/object/file.rs:204:55
stack backtrace:
   0:     0x5567bf58959d - std::backtrace_rs::backtrace::libunwind::trace::h22893a5306c091b4
                               at /rustc/fe5b13d681f25ee6474be29d748c65adcd91f69e/library/std/src/../../backtrace/src/backtrace/libunwind.rs:93:5
   1:     0x5567bf58959d - std::backtrace_rs::backtrace::trace_unsynchronized::h29c3bc6f9e91819d
                               at /rustc/fe5b13d681f25ee6474be29d748c65adcd91f69e/library/std/src/../../backtrace/src/backtrace/mod.rs:66:5
   2:     0x5567bf58959d - std::sys_common::backtrace::_print_fmt::he497d8a0ec903793
                               at /rustc/fe5b13d681f25ee6474be29d748c65adcd91f69e/library/std/src/sys_common/backtrace.rs:66:5
   3:     0x5567bf58959d - <std::sys_common::backtrace::_print::DisplayBacktrace as core::fmt::Display>::fmt::h9c2a9d2774d81873
                               at /rustc/fe5b13d681f25ee6474be29d748c65adcd91f69e/library/std/src/sys_common/backtrace.rs:45:22
   4:     0x5567bf5ab98c - core::fmt::write::hba4337c43d992f49
                               at /rustc/fe5b13d681f25ee6474be29d748c65adcd91f69e/library/core/src/fmt/mod.rs:1194:17
   5:     0x5567bf582f51 - std::io::Write::write_fmt::heb73de6e02cfabed
                               at /rustc/fe5b13d681f25ee6474be29d748c65adcd91f69e/library/std/src/io/mod.rs:1655:15
   6:     0x5567bf58b0c5 - std::sys_common::backtrace::_print::h63c8b24acdd8e8ce
                               at /rustc/fe5b13d681f25ee6474be29d748c65adcd91f69e/library/std/src/sys_common/backtrace.rs:48:5
   7:     0x5567bf58b0c5 - std::sys_common::backtrace::print::h426700d6240cdcc2
                               at /rustc/fe5b13d681f25ee6474be29d748c65adcd91f69e/library/std/src/sys_common/backtrace.rs:35:9
   8:     0x5567bf58b0c5 - std::panicking::default_hook::{{closure}}::hc9a76eed0b18f82b
                               at /rustc/fe5b13d681f25ee6474be29d748c65adcd91f69e/library/std/src/panicking.rs:295:22
   9:     0x5567bf58ad79 - std::panicking::default_hook::h2e88d02087fae196
                               at /rustc/fe5b13d681f25ee6474be29d748c65adcd91f69e/library/std/src/panicking.rs:314:9
  10:     0x5567bf58b612 - std::panicking::rust_panic_with_hook::habfdcc2e90f9fd4c
                               at /rustc/fe5b13d681f25ee6474be29d748c65adcd91f69e/library/std/src/panicking.rs:698:17
  11:     0x5567bf58b4f7 - std::panicking::begin_panic_handler::{{closure}}::he054b2a83a51d2cd
                               at /rustc/fe5b13d681f25ee6474be29d748c65adcd91f69e/library/std/src/panicking.rs:588:13
  12:     0x5567bf589a54 - std::sys_common::backtrace::__rust_end_short_backtrace::ha48b94ab49b30915
                               at /rustc/fe5b13d681f25ee6474be29d748c65adcd91f69e/library/std/src/sys_common/backtrace.rs:138:18
  13:     0x5567bf58b229 - rust_begin_unwind
                               at /rustc/fe5b13d681f25ee6474be29d748c65adcd91f69e/library/std/src/panicking.rs:584:5
  14:     0x5567bf263a33 - core::panicking::panic_fmt::h366d3a309ae17c94
                               at /rustc/fe5b13d681f25ee6474be29d748c65adcd91f69e/library/core/src/panicking.rs:143:14
  15:     0x5567bf263b23 - core::result::unwrap_failed::hddd78f4658ac7d0f
                               at /rustc/fe5b13d681f25ee6474be29d748c65adcd91f69e/library/core/src/result.rs:1785:5
  16:     0x5567bf2830ce - <nbd_rs::object::file::FileBackend as nbd_rs::object::SimpleObjectStorage>::end_operations_on_object::hd06d901593d8a637
  17:     0x5567bf292dc1 - <nbd_rs::block::raw::RawBlock as nbd_rs::block::BlockStorage>::close::hd0c666b12534f51a
  18:     0x5567bf268b40 - nbd_rs::nbd::session::NBDSession::handle_request::h5ae243202b3b2d29
  19:     0x5567bf264648 - nbd_rs::nbd::session::NBDSession::handle::h67ffd1e11a6b43c6
  20:     0x5567bf29f248 - nbd_rs::nbd::server::NBDServer::listen::h9879573fccbd7eb9
  21:     0x5567bf27c6fe - nbd_rs::main::h15939c634f77585c
  22:     0x5567bf29b873 - std::sys_common::backtrace::__rust_begin_short_backtrace::h3f3885464d7d69a2
  23:     0x5567bf29c7e9 - std::rt::lang_start::{{closure}}::h8696a8245012fdd2
  24:     0x5567bf57d90e - core::ops::function::impls::<impl core::ops::function::FnOnce<A> for &F>::call_once::had4f69b3aefb47a8
                               at /rustc/fe5b13d681f25ee6474be29d748c65adcd91f69e/library/core/src/ops/function.rs:259:13
  25:     0x5567bf57d90e - std::panicking::try::do_call::hf2ad5355fcafe775
                               at /rustc/fe5b13d681f25ee6474be29d748c65adcd91f69e/library/std/src/panicking.rs:492:40
  26:     0x5567bf57d90e - std::panicking::try::h0a63ac363423e61e
                               at /rustc/fe5b13d681f25ee6474be29d748c65adcd91f69e/library/std/src/panicking.rs:456:19
  27:     0x5567bf57d90e - std::panic::catch_unwind::h18088edcecb8693a
                               at /rustc/fe5b13d681f25ee6474be29d748c65adcd91f69e/library/std/src/panic.rs:137:14
  28:     0x5567bf57d90e - std::rt::lang_start_internal::{{closure}}::ha7dad166dc711761
                               at /rustc/fe5b13d681f25ee6474be29d748c65adcd91f69e/library/std/src/rt.rs:128:48
  29:     0x5567bf57d90e - std::panicking::try::do_call::hda0c61bf3a57d6e6
                               at /rustc/fe5b13d681f25ee6474be29d748c65adcd91f69e/library/std/src/panicking.rs:492:40
  30:     0x5567bf57d90e - std::panicking::try::hbc940e68560040a9
                               at /rustc/fe5b13d681f25ee6474be29d748c65adcd91f69e/library/std/src/panicking.rs:456:19
  31:     0x5567bf57d90e - std::panic::catch_unwind::haed0df2aeb3fa368
                               at /rustc/fe5b13d681f25ee6474be29d748c65adcd91f69e/library/std/src/panic.rs:137:14
  32:     0x5567bf57d90e - std::rt::lang_start_internal::h9c06694362b5b80c
                               at /rustc/fe5b13d681f25ee6474be29d748c65adcd91f69e/library/std/src/rt.rs:128:20
  33:     0x5567bf27c9c2 - main
  34:     0x7f33dd3bb09b - __libc_start_main
  35:     0x5567bf263cda - _start
  36:                0x0 - <unknown>