(shell-command-default-error-buffer): Renamed from
shell-command-on-region-default-error-buffer. (shell-command-on-region): Mention in echo area when there is some error output. Mention success or failure, too. Accumulate multiple error outputs going forward, with formfeed in between. Display the error buffer when we have put something in it. (shell-command): Add the ERROR-BUFFER argument feature.
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