notmuch setup/new: Propagate failure from notmuch_database_set_timestamp

With some recent testing, the timestamp was failing, (overflowing
the term limit), and reporting an error, but the top-level notmuch
command was still returning a success return value.

I think it's high time to add a test suite, (and the code base is
small enough that if we add it now it shouldn't be *too* hard to
shoot for a very high coverage percentage).