emacs: Use async process helper for search

emacs: Use async process helper for search

Previously, search started the async notmuch process directly.  Now,
it uses `notmuch-start-notmuch'.  This simplifies the process sentinel
a bit and means that we no longer have to worry about errors
interleaved with the JSON output.

We also update the tests of Emacs error handling, since the error
output is now separated from the search results buffer.