add error to call model (#539)

* add error to call model

closes #331

previously, for async this error was being masked completely even if it was
something useful like the image not existing. for sync, the error was returned
in the http request but now it's also being stored. this error itself can
cover a lot of landscape, it could be an error in getting a slot, pulling an
image, running a container, among other things. anyway, no longer being
masked. we can likely improve it in certain cases we run into in the future,
but it's open ended at the moment and not being masked like some errors in
sync http request returns (503 non-models.APIError) for now.

* tucks in callTrigger stuff to keep api clean
* adds swagger
* adds migration
* adds tests for datastore and agent to ensure behavior

* pull images before tests are ran

* gofmt migrations file

docs/swagger.yml

@@ -646,6 +646,10 @@ definitions:
         type: string
         description: Call execution status.
         readOnly: true
+      error:
+        type: string
+        description: Call execution error, if status is 'error'.
+        readOnly: true
       app_name:
         type: string
         description: App name that is assigned to a route that is being executed.