core: make status code mandatory in abort
core: make status code mandatory in abort

The default of None does not work and raises a KeyError, so there's no point
having this as a default. I don't think there is a correct "default" abort
status code we could use either, so let force the user specifies what it wants.

Change-Id: I39e74e95a82a4bb3c22305809edabccc910cd073

jdandrea/pecan,jdandrea/pecan,StackStorm/pecan,pecan/pecan,ryanpetrello/pecan,pecan/pecan,StackStorm/pecan,ryanpetrello/pecan