Handle non-oai response during Harvest

[JPrevost]

This is starting to get a bit messy, but I think this ends the work in this general area so maybe it's okay.

closes #214

[coveralls]

Coverage decreased (-0.04%) to 53.1% when pulling f5d5a59 on 214_non_oai_returned_at_harvest into 36af447 on master.

[richardrodgers]

Yea, I'm not wild about all that counter/break stuff - but I guess we can refactor later
👍

[JPrevost]

Yeah, it's definitely a bit of code smelly (to be generous) as is but I didn't have an immediate better idea of how to detect a valid OAI-PMH XML file without switching to a DOM parsing model. Like you said, we can come back and refactor if/when one of us has a better idea or if we notice this falling apart in real world scenarios.