Add reasons for requires in contracts

[cuongdo]

Checklist:

• [x] Code contains relevant tests for the problem you are solving
• [x] Ensure all new and existing tests pass
• [x] Update any relevant READMEs and docs

Description:

Add reason strings to requires

This makes the cause of an EVM revert much clearer to developers. I've tried to keep the strings short but unambiguous within each function.

I've had to increase the gas limit for contract test deploys, because it now takes ~5 million gas (compared to ~4 million previously) to deploy the marketplace contract. This corresponds to a difference in deployment cost of ~$1.31 USD. The small increase is more than offset by reduction in developer debugging time. The lowest gas limit I've seen from Rinkeby, Ropsten, and mainnet is 7 million, so we're OK there.

I also haven't touched contracts/test-alt/contracts

Takes care of #457 for Origin contract code

OpenZeppelin code remains uninstrumented until we vendor it.

[cuongdo]

cc @wanderingstan is this what you were thinking?

[wanderingstan]

Sorry this is late, but YES! Thank you @cuongdo !