bug#67637: shell-command-default-error-buffer loss of warning

[Dan Jacobson]

(Never mind https://debbugs.gnu.org/cgi/bugreport.cgi?bug=67542 .
I have found the real problem.)

$ emacs -Q --eval '(setq shell-command-default-error-buffer 
"shell-command-errors")'
M-! ls zzzzzzzz
(Shell command failed with code 2 and some error output to the 
"shell-command-errors" buffer)

OK, we now grow to expect that we will get helpful notification that
something about "some error output to the "shell-command-errors"
buffer". That's great.

Until one day when we instead do
M-! ls .emacs zzzzzzzz #one file that exists and one that doesn't

In this case the bug steps in:
Even though there indeed is
"some error output to the "shell-command-errors"
but there is no message in the minibuffer about it!

That indeed there is also some STDOUT produced does *not* mean
we somehow are in some party mood and no longer care about the fact that
indeed the same complete
"Shell command failed with code 2 and some error output to the 
"shell-command-errors" buffer"
is 100% still true.

One might think that "oh, that's just the one line successful output
covering up the error message in the minibuffer." But the same problem
occurs with
M-! ls . zzzzzzzz