user left uninformed of shell-command errors

[Dan Jacobson]

this command will print out a helpful message in the minibuffer about
its failure

(shell-command ":;false" nil nil)

this command won't:

(shell-command "echo;false" nil nil)

you might think oh no big deal, but when output goes beyond the
screen, e.g.

(dired-do-shell-command "gunzip -qc * | tar xvf -" nil (quote 
("gkrellm-1.2.4.tar.gz")))

and something like

gunzip: gkrellm-1.2.4.tar.gz: unexpected end of file
tar: Unexpected EOF in archive
tar: Error exit delayed from previous errors

happens, then, all you see is the good news [untarred filenames] but
the bad news is only available if you insist on checking to the bottom
of that buffer.

So emacs could have told you about an error, but doesn't because for
some reason, it only produces
(Shell command succeeded with no output)
(Shell command failed with no output)
and in the case it failed with output no warning is produced.

Now consider the case of
(shell-command "echo >&2;false" nil nil)
not only has there been stderr output, we also have a failure
condition, but the user doesn't see any of it.

I'd say not only talk about problems in the minibuffer, also the
modeline should have a, perhaps revers video "FAILED", etc.

Note that my examples are mostly M-!, but this also affects dired-x !
stuff.  Emacs 20.7.
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