lab mr create: Check file exists (-F/-f options)
lab mr create: Check file exists (-F/-f options)

Currently, when the file specified with the -F or -f options does not
exist, lab outputs a very generic error that is not helpful to users

[prarit@prarit kernel-test]$ ~/git-kernel/github/lab/lab mr create
2021/09/22 07:14:40 ERROR: mr_create.go:289: empty MR message

Check to see if the file exists and report a better error if the file is
not present.  For example,

[prarit@prarit kernel-test]$ ~/git-kernel/github/lab/lab mr create -F /tmp/non-existent.file
2021/09/22 07:15:02 ERROR: mr_create.go:244: file /tmp/non-existent.file cannot be found

Fixes #742

Signed-off-by: Prarit Bhargava <fe4655370dc87a32329d591d6e8472329333dfad@redhat.com>

zaquestion/lab,zaquestion/lab