Make the generator output easier-to-understand error message for undefined types.

BUG=788704

Change-Id: Iedd0fb694e88db9a492f9b670f1e1fd177120a10
Reviewed-on: https://chromium-review.googlesource.com/802402Reviewed-by: Ken Rockot <rockot@chromium.org>
Commit-Queue: Yuzhu Shen <yzshen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#520822}

mojo/public/tools/bindings/pylib/mojom/generate/pack.py

@@ -56,7 +56,8 @@ class PackedField(object):
       # TODO(mpcomplete): what about big enums?
       return cls.kind_to_size[mojom.INT32]
     if not kind in cls.kind_to_size:
-      raise Exception("Invalid kind: %s" % kind.spec)
+      raise Exception("Undefined type: %s. Did you forget to import the file "
+                      "containing the definition?" % kind.spec)
     return cls.kind_to_size[kind]
 
   @classmethod