A couple minor changes to error handling in startup:
1. Don't dump full help/usage text when the only problem is `runc` wasn't started under
root privileges
2. Check for rootfs and make error clear to user when it doesn't exist
3. Change fatal to logrus.Fatal to get nicer output with simple error
message

Docker-DCO-1.1-Signed-off-by: Phil Estes <estesp@linux.vnet.ibm.com> (github: estesp)