(find_mount_point): Emit a diagnostic for each

(find_mount_point): Emit a diagnostic for each
failed syscall, rather than relying on caller to do that.
The caller couldn't do a good job, anyhow -- too many different
ways to fail (each with a different referent).
Give a diagnostic upon failed save_cwd, now that that function
no longer calls `error'.
(show_point): Don't diagnose find_mount_point's errors, now that
it handles them itself.