Wrong 404 behavior on taxonomy listing

No results ≠ 404.

404 must be returned when term does not exist. Not when not results are returned. 

If no results, template says "No posts for {{ term }}".
If term does not exist, then return a 404.
Merge remote-tracking branch 'upstream/master'
Changed abort message.
Test slug against array keys as options can be a slug => Label array
Refine taxonomy behavior:
- When 'behaves_like' like 'tags', return 404 when no content has been found
- When 'behaves_like' like 'categories' or 'grouping', return 404 only if taxonomy slug does not exist
Merge pull request #2310 from nlemoine/master

 Wrong 404 behavior on taxonomy listing