Fix InvalidUsage, was missing parameter

Created by: Samwalton9

I'll still post PRs for most things that touch Flask code since I have little to no experience of it, especially while I don't have access to a fully functioning version of the tool.

Anyway, looks like you took this code from http://flask.pocoo.org/docs/0.12/patterns/apierrors/ but tje 'payload' parameter was missing so it errors out if you have, for example, a blank edit summary. Think this should fix, but untested.