Mark a buildset FAILED if anything was worse than WARNINGS

In particular, if a build's result was EXCEPTION, this will cause the
whole buildset to be represented as FAILED in the database.  This has
implications for downstream schedulers, but not for the web status.

Fixes #969

master/buildbot/db/connector.py

@@ -955,8 +955,10 @@ def _check_buildset(self, t, bsid, now):
             if not complete:
                 # still waiting
                 is_complete = False
-            if r == FAILURE:
-                bs_results = r
+            # mark the buildset as a failure if anything worse than
+            # WARNINGS resulted from any one of the buildrequests
+            if r not in (SUCCESS, WARNINGS):
+                bs_results = FAILURE
         if is_complete:
             # they were all successful
             q = self.quoteq("UPDATE buildsets"