text-based mount.nfs: Add text-based error reporting function

The mount_errors() function prints an error based on what just happened in
the user-space RPC library.  This is meaningless for text-based mounts,
since they don't use the RPC library for most things.

Add a new error printing function that the text-based logic can use to
report an error.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Neil Brown <neilb@suse.de>