Improved error message for when branch not whitelisted in features yaml

Change-Id: I587cd38ad2207291c984b357b4a27128014d509c

test-matrix.py

@@ -58,7 +58,7 @@ def normalize_branch(branch):
             # projects are developing master.
             branch = GRID['branches']['default']
     if branch not in ALLOWED_BRANCHES:
-        LOG.error("unknown branch name %s" % branch)
+        LOG.error("branch not allowed by features matrix: %s" % branch)
         sys.exit(1)
     return branch