edit: Report unit key 'U' failure more nicely, and avoid oops

edit: Report unit key 'U' failure more nicely, and avoid oops

Two failure modes:

* When the argument is negative, edit() attempts an invalid write,
  triggering an oops.  edit() detects the write failure, and returns
  RET_FAIL.

  Detect the negative argument instead, and return RET_SYN, like we do
  for other invalid edit arguments.

* When the argument requires the table to be extended, but the
  extension fails, edit() returns RET_FAIL.

  Additionally report the failure to the player.

Signed-off-by: Markus Armbruster <armbru@pond.sub.org>