Added error handeling to api.get_token

`get_token` would make a request to /ouath2/token and then assume that the
response from that call was valid.  Added a simple check to raise an error
if the request failed.

wepay/api.py

@@ -122,5 +122,13 @@ def get_token(
         if callback_uri:
             params.update({'callback_uri': callback_uri})
         response = self.call('/oauth2/token', params)
+
+        # The call to /oauth2/token should return an access_token
+        # if the access_token was not returned, then an error occured
+        # we need to raise this error, 
+        # otherwise this will die when trying to use the 'acess_token' field
+        if 'access_token' not in response:
+            raise WePayError(response['error'], response['error_code'], response['error_description'])
+
         self.access_token = response['access_token']
         return response