emacs: Do not pass stderr of notmuch reply to JSON parser

emacs: Do not pass stderr of notmuch reply to JSON parser

Sometimes, notmuch reply outputs something to stderr, for example:
"Failed to verify signed part: Cannot verify multipart/signed part:
unsupported signature protocol". When this happens, replying in emacs
fails, because emacs cannot parse the error message as JSON.

This patch causes emacs to ignore stderr when reading reply from
notmuch.