Functions in SafeMath contract overloaded to accept custom error messages

[elch-yan]

Fixes #1827
Overloaded counterparts of the existing functions created, and original functions calls redirected to them.

[nventuro]

Great work @elch-yan, thanks! Do you think you could also tackle the following?

1. Add an entry to the CHANGELOG file with this new feature
2. Add some of these custom checks to relevant contracts, e.g. ERC20._transfer, with a custom error message.

Regarding scope, I'm not sure how useful the overloads for add, mul div and mod are, since those are often used to check for actual arithmetic errors (overflows or division by zero) as opposed to e.g. sufficient balance. Perhaps we should limit this to sub? @frangio

[elch-yan]

Thank you @nventuro. Sure I'll update CHANGELOG and will add relevant checks to ERC20._transfer.
I was also been thinking about usefulness of this for the remaining 4 methods, and I guess that it might be somewhat useful for div and mod for 0 checks (spliting funds for example), but guess you're right about mul and add.

[nventuro]

and I guess that it might be somewhat useful for div and mod for 0 checks (spliting funds for example)

Ohh, interesting! There are some examples of this in PaymentSplitter, TokenVesting and IncreasingTimeCrowdsale.

[elch-yan]

@nventuro I've made an entry in CHANGELOG, did some modifications in token contracts and related tests, regarding custom errors.
I looked at PaymentSplitter, TokenVesting and IncreasingTimeCrowdsale and I guess that in their case divisor can never become a 0.
I'd be happy to hear your opinion regarding my changes and about whether or not you'd like me to keep overloads for SafeMath functions other than sub.

[frangio]

I think we should only add those that we currently need, which seem to be sub, div and mod, from what @nventuro said.

[elch-yan]

All done!

[frangio]

Thanks @elch-yan!

One last thing: we would like to change the new error messages slightly to make them more aligned with the rest of the library. Could you apply the following changes?

• "transfer of tokens exceeding allowance" → "transfer amount exceeds allowance"
• "transfer of tokens exceeding balance" → "transfer amount exceeds balance"/"transfer amount exceeds supply" (depending whether the variable in the operation is the total supply)
• "decrease of higher allowance than granted" → "decreased allowance below zero"

Those which say "transfer" also apply to "burn".

[frangio]

Awesome @elch-yan! Thanks!

[nventuro]

Great work @elch-yan, thanks a lot!