emacs: Display any unexpected output from notmuch search

Rather than silently swallowing unexpected output, the emacs interface will now
display it. This will allow error messages to actually arrive at the emacs
interface (though not in an especially pretty way). This also allows for easier
investigation of the inadvertent swallowing of search results that span page
boundaries (as demonstrated by the recent added emacs-large-search-buffer test).

The page-boundary bug has been present since a commit from 2009-11-24:
93af7b574598637c2766dd1f8ef343962c9a8efb

Many thanks to Thomas Schwinge for tracking that bug down and
contributing the test for it.