Add revert with reason functionality for mock contracts

[PaulRBerg]

Not being able to revert with a reason on a mock contract is prohibitively limiting.

Say you have contract A and contract B. The latter depends on the former. Say that the former throws specific errors depending on the state of the properties in the two contracts. If I mock contract A, I have to modify a lot of tests to revert with "Mock revert" instead of my bespoke error messages.

This PR does the following:

• Adds a new revertReason property in the MockCall struct
• Changes the signature of the __waffle__mockReverts function, by adding a second parameter revertReason
• Adds a new function revertWithReason in the object returned by the stub function - which makes my change backwards-compatible!
• Modifies the tests so that they work with the updated __waffle__mockReverts function

[marekkirejczyk]

@PaulRBerg Good stuff, merging. Would add a PR with docs?

[marekkirejczyk]

Released in 3.0.3

[PaulRBerg]

Thanks for merging, Marek!

Would add a PR with docs?

I could do this, in case you haven't done it in the meantime?

[marekkirejczyk]

Released in Waffle 3.1.0. Didn't have a chance to work on docs yet. Would love to get some help :)

[PaulRBerg]

Released in Waffle 3.1.0. Didn't have a chance to work on docs yet. Would love to get some help :)

Done: #366.