Bug 1738684 - Avoid the warnings collector from eating build output. …

…r=firefox-build-system-reviewers,ahochheiden

If for some reason the warning collector throws an exception, we ignore
it, which is fine, but we also end up not setting the message for the
BuildOutputResult, which means the line is not displayed.

It's better for the warning collector to miss a warning than for the
output to pretend the line wasn't there to begin with.

Differential Revision: https://phabricator.services.mozilla.com/D192046

python/mozbuild/mozbuild/controller/building.py

@@ -355,10 +355,10 @@ def on_line(self, line):
             return BuildOutputResult(None, False, None)
 
         warning = None
+        message = line
 
         try:
             warning = self._warnings_collector.process_line(line)
-            message = line
         except Exception:
             pass