emacs: Refactor subprocess stderr propagation

emacs: Refactor subprocess stderr propagation

Load subprocess error output to a string in the callers, and propagate
the error messages as a string parameter instead of a path to file
names.

Required to be able to avoid using temporary files for subprocess
error output.

* notmuch-lib.el: Update notmuch-check-async-exit-status,
  notmuch-check-exit-status: accept an err parameter instead of
  err-file; shift the responsibility of loading error messages from
  files up the call stack.