Add error messages for require statements

[paulinablaszk]

I think we should add error messages for all require (and assert) statements in contracts. What do you think about the format of reason strings? I think it should be consistent with other OpenZeppelin contracts

[abcoathup]

I agree that the format should be consistent with other OpenZeppelin contracts.

Should we wait until #117 is done and then update or do it as part of that? I am keen to get the upgrade to Solidity 0.5 complete.

[vibern0]

I don't have good experience with require error messages on ropsten. How about you guys?
And this seems to be the solution (or the problem) ethereum/go-ethereum#18360
Also, we should wait until #117 is merged and then move on with everything else, in my opinion.

[paulinablaszk]

I agree we have to wait for the update to Solidity 0.5, I was thinking about adding error messages during #177 but I didn't want to mix these issues.
I think that even if error messages are not supported by all clients, it might be a
example of good practices for people learning Solidity, because it is helpful in the development process

[xaler5]

@paulinablaszk are there still requires without messages ?