coding system warning should say what file it is talking about

[Dan Jacobson]

There is a buffer I want to show you, called *Warning*, but you have
made very sure that I can only see it and cannot find it later in the
buffer list to quote it in email.

OK, I managed to grab its contents by C-x o C-x o C-x h <escape> w:

  These default coding systems were tried:
    chinese-big5-unix
  However, none of them safely encodes the target text.

  Select one of the following safe coding systems:
    iso-8859-9 iso-8859-8 iso-8859-4 iso-8859-3 iso-8859-2 iso-8859-1

etc. which extend off the screen, with no way for the user to scroll
the contents while being prompted in the minibuffer.

Anyway, the main reason I'm writing is that at the top of the message
it should say which file it is talking about. As we encountered it
upon using "C-x s !" for crying out loud...
emacs-version"21.3.1"