emacs: View the output of pipe command when it fails

emacs: View the output of pipe command when it fails

Previously, the user didn't know whether the pipe command succeeded or
not. It was only possible to find it out by manually inspecting
the work done (or not done) by the command or by manually switching to
*notmuch-pipe* buffer and determine it from command output. For this
the user had to first find the text corresponding to the last run of
pipe command as the buffer accumulated the output from all pipe commands.

This patch changes the following. The *notmuch-pipe* buffer is erased
before every pipe command so it contains only the output from the last
command. Additionally, when the command failed, the *notmuch-pipe* buffer
is shown and an error message is displayed.
with the output of pipe command.