BM-871: Always revert on invalid context

[capossele]

Addressing @nategraf comments mentioned in #596 :

1. Adjust the comment at the top of this function to reflect the existence of this kind of weird edge case around a request becoming fulfilled without payment being sent.
2. Document (and double check) the invariant that the first ProofDelivered event should coincide with a Fulfilled event in all cases.
3. Revert on !context.valid. This is actually a separate bug I am just noticing now. The issue is that, in particular for smart contract signed requests, the signature check on a request happens in the priceReqeust function. The current implementation using return instead of revert creates an issue where a third party can cancel someone else request by crafting a request with the same ID and fulfilling it before any lock occurs. They can do so without the request ever being signed.

[nategraf]

L591 in this file is an inaccurate comment at this point.

[nategraf]

I pushed fcd7ba3 with a couple of code comments, and verified that the invariant I describe currently holds.