Hint at missing role in revert reason for AccessControl rejection

[frangio]

It would be useful to notify clients of what role is necessary to call a function protected by AccessControl. We can include this information in the revert reason in a standard and machine-parseable format.

Note that we don't have a built in function or modifier to check if an account has a role. Users are currently encouraged to use require(hasRole(MINTER_ROLE, msg.sender)), so we would need to add this new modifier or function in order to standardize the revert reason hint.

[Amxx]

+1 for the modifier