Bug 1312209 - Make some small fixes to runtests.py in case a test manifest isn't found; r=ted

Before these changes we get a bunch of Python errors.
Now we get a human readable warning saying the test
manifest does not exist.

testing/mochitest/runtests.py

@@ -1300,6 +1300,7 @@ toolbar#nav-bar {
         if self._active_tests:
             return self._active_tests
 
+        tests = []
         manifest = self.getTestManifest(options)
         if manifest:
             if options.extra_mozinfo_json:
@@ -1429,7 +1430,8 @@ toolbar#nav-bar {
             if os.path.exists(masterPath):
                 manifest = TestManifest([masterPath], strict=False)
             else:
-                self._log.warning(
+                manifest = None
+                self.log.warning(
                     'TestManifest masterPath %s does not exist' %
                     masterPath)