end2end: Improve output when subprocess exits

We don't really have the "infrastructure" set up yet to have proper log
output at that point, but we can always just show the log output as part
of the exception.

tests/end2end/fixtures/testprocess.py

@@ -234,7 +234,7 @@ class Process(QObject):
             if not self.is_running():
                 # _start ensures it actually started, but it might quit shortly
                 # afterwards
-                raise ProcessExited()
+                raise ProcessExited('\n' + _render_log(self.captured_log))
 
             if blocker.signal_triggered:
                 self._after_start()