Handling GAE BlobStore Exception via webapp2 handler
The 413 is generated by the App Engine infrastructure; the request neve reaches your app, so it's impossible to handle this condition yourself.
I have been banging my head around on this issue for a bit and have not come up with a solution.  I am attempting to trap the exception UploadEntityTooLargeEntity.  This exception is raised by GAE when 2 things happen.

Set the max_bytes_total param in the create_upload_url:
self.template_values['AVATAR_SAVE_URL'] = blobstore.create_upload_url('/saveavatar,
                                                                                   max_bytes_total= 524288)
Attempt to post an item that exceeds the max_bytes_total.

I expect that, since my class is derived from RequestHandler that my error() method would be called.  Instead I am getting a 413 screen telling me the upload is too large.
My request handler is derived from webapp2.RequestHandler.  Is it expected that GAE will work with the error method derived from webapp2.RequestHandler?  I'm not seeing this in GAE's code but I can't imagine there would be such an omission.