emacs: Don't report CLI signals sent by Emacs as errors

Previously, when the user killed the search buffer before the CLI
search process had completed, we would report the signal sent by Emacs
to kill the CLI to the user as an error.  Fix this by only reporting
error exits if the process buffer is still live.  We still report
stderr output regardless in case stderr output was relevant to why the
user killed the search buffer (such as a wrapper script being stuck).