emacs: poll: return useful errors when poll fails.

Previously poll called from emacs would fail silently. This makes it
return a useful error message.

In the non-deprecated case of notmuch new and appropriate hooks, it
uses notmuch-call-notmuch-process which gives an error and
additionally puts the stdout/stderr etc in the *Notmuch errors*
buffer.

In the deprecated case of a custom poll script it only returns an
error message.

Commit based on a bug report, and a potential fix, by Ketil Malde.