Committed in [15791] and merged in [15792-15793].
After the changes, the following would be logged.
2017-04-17 15:26:08,669 Trac[chrome] WARNING: Error with navigation contributor AboutModule: TracError: xxx
2017-04-17 15:26:36,242 Trac[chrome] ERROR: Error with navigation contributor AboutModule:
Traceback (most recent call last):
File "/src/tracdev/svn/branches/1.0-stable/trac/web/chrome.py", line 786, in prepare_request
contributor.get_navigation_items(req) or []:
File "/src/tracdev/svn/branches/1.0-stable/trac/about.py", line 47, in get_navigation_items
raise ValueError('xxx')
ValueError: xxx
