Improve error message for missing NFS share config

If FLAGS.nfs_share_config is not defined, the error will reference
the name of the config option to set.

If the nfs_share_config path doesn't exist, the error will reference
the path that was configured.

This will hopefully simplify troubleshooting of NFS configuration.

Change-Id: Ica6a60a63330554dc880f36962f3a081926ce5cf
Merge "Improve error message for missing NFS share config"