fix(sales): do not crash when retrieving request fails

[markspanbroek]

Fixes #1247

[emizzle]

This is great, thank you Mark. I believe think this may be a partial fix for #1247. It'll prevent the node from crashing when a request cannot be found, but it doesn't address the root cause of why the request could not be found in the contract. Therefore, I'd be hesitant to close #1247 when merging this.

[markspanbroek]

I looked through the nim and solidity code once more, but can't think of a reason why getting the storage request returns a Marketplace_UnknownRequest error after we just got an event about the same storage request, other than that the JSON-RPC node might be looking at old state. If Geth were to have a race condition in which it sends out the event just before updating the current state, then this could happen. I'm speculating here, but given that we've seen this error while running against a very old Geth, I don't really want to dig into that codebase to find out more.