Dump reviewers set when failing _check_reviewers

I've hit this assert a few times, seeing the output of the set usually
helps highlight quickly how my command-line arguments are broken.

Bug:
Change-Id: I8b80b635df91f2707a599a33ab9b4959b3c76614
Reviewed-on: https://chromium-review.googlesource.com/535834Reviewed-by: Andrii Shyshkalov <tandrii@chromium.org>
Commit-Queue: Gabriel Charette <gab@chromium.org>

owners.py

@@ -185,7 +185,7 @@ class Database(object):
 
   def _check_reviewers(self, reviewers):
     _assert_is_collection(reviewers)
-    assert all(self.email_regexp.match(r) for r in reviewers)
+    assert all(self.email_regexp.match(r) for r in reviewers), reviewers
 
   def _is_obj_covered_by(self, objname, reviewers):
     reviewers = list(reviewers) + [EVERYONE]