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.