emacs: notmuch-check-exit-status bugfix

This function prints diagnostic information in the event of an
error. However, one of the callers has an optional :stdin-string
keyword argument. This causes the error printing routine to error
itself.

Rather than reach notmuch-check-exit-status about the possible keyword
arguments (currently only one but could be more in the future) this
commit just tells notmuch-check-exit-status how to print non-string arguments.