Bug 1057377 - Always create a bug_suggestions artefact; r=mdoglio

The UI expects there to be one bugs_suggestions item per log error
line. Previously if there was no valid search term extracted from a log
error line, an item would not be added to the bugs_suggestion artefact.
Bug 1057377 - Return search_term even if it is None; r=mdoglio

Prior to returning, search_term would have already been set to the
whole log line if suitable, so if search_term is None we shouldn't
return the whole log error line, otherwise the search term blacklist
won't have any effect.
Bug 1057377 - Return None not the empty string if no error_line; r=mdoglio

This function returns None elsewhere, so let's be consistent.
Bug 1057377 - Return None if original & fallback search terms unsuitable; r=mdoglio

Prior to this patch, if we found an initial search term, but that
search term was unsuitable (eg on blacklist) and the whole log line
fallback *also* resulted in an unsuitable search term, then we would
still return the initial search term, rather than None.
Merge pull request #191 from mozilla/1057377-fix-blacklist

Bug 1057377 - Bug suggestions & blacklist fixes