More context when _load_codereview_impl asserts

Bug: 908466
Change-Id: I715633c64fff343d07dbcf50c261e686d77e8c44
Reviewed-on: https://chromium-review.googlesource.com/c/1351511
Reviewed-by: Andy Perelson <ajp@chromium.org>
Reviewed-by: Edward Lesmes <ehmaldonado@chromium.org>
Commit-Queue: Joe Mason <joenotcharles@google.com>

git_cl.py

@@ -1110,7 +1110,9 @@ class Changelist(object):
 
   def _load_codereview_impl(self, codereview=None, **kwargs):
     if codereview:
-      assert codereview in _CODEREVIEW_IMPLEMENTATIONS
+      assert codereview in _CODEREVIEW_IMPLEMENTATIONS, (
+          'codereview {} not in {}'.format(codereview,
+                                           _CODEREVIEW_IMPLEMENTATIONS))
       cls = _CODEREVIEW_IMPLEMENTATIONS[codereview]
       self._codereview = codereview
       self._codereview_impl = cls(self, **kwargs)