At the moment, if a test file throws an exception when loaded (for example it tries to import from a URL that doesn't exist and throws NS_ERROR_FILE_NOT_FOUND), the log shows no error message and not even the file name.
This can be improved by sending better information to the log writer. I can't actually get just the exception message itself (the exception object is weird), so I'll send the whole string representation.
SUMMARY-UNEXPECTED-FAIL | testCategoryColors.js | testCategoryColors.js::<TOP_LEVEL>
EXCEPTION: [Exception... "File error: Not found" nsresult: "0x80520012 (NS_ERROR_FILE_NOT_FOUND)" location: "JS frame :: file:///home/geoff/mozilla/mozilla-central/obj-artifact/_tests/mozmill/stage/preferences/testCategoryColors.js :: <TOP_LEVEL> :: line 9" data: no]
at: testCategoryColors.js line 9