mount.nfs: rename mount_errors()

The function mount_errors() actually reports RPC errors generated by the
user-land RPC library.  We're about to add a similar function for reporting
system call errors via errno, so rename mount_errors() to be more specific
about what it does.

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