Emit an event on App when a state creator throws an error
Change App to emit errors it has handled, in case other entitities would like to know about them
Fix broken AppTester tests after changing App to emit error events for handled errrors
Fix AppEvent docstring
Fix app event and error docstrings
Merge branch 'feature/issue-74-errors-started-in-app-dont-get-rethrown-with-apptester' into develop