Error handling in ERC20 and ERC721

[princesinha19]

I did some error handling in ERC20 and ERC721. In every require, I added some messages so that developer or end user can easily understand what's wrong.

Fixes #1701

[princesinha19]

@frangio @nventuro Please have a look at this PR. I added message in require statement and also made some changes in ERC20.sol and ERC721.sol. Please let me know If you guys think some changes are need to be done.

[nventuro]

Hello @princesinha19, thank you for contributing!

We've received multiple requests for this (ever since #888) and will be working on it shortly: see my related comment here for more relevant information.

[princesinha19]

okay cools @nventuro

[nventuro]

Please see this post in our forum for discussion regarding revert reasons, and share your thoughts!

[nventuro]

@princesinha19 I opened #1709 with some guidelines regarding message format, among other things, including some examples. Could you review it and update your PR accordingly? Thanks!

[nventuro]

Thank you for your initiative @princesinha19! I left some comments guiding some of the messages, along with a couple questions. Please review all reason strings according to these. Also, don't forget to append a prefix with the contract name (e.g. 'ERC20: ').

Thanks again!

[princesinha19]

thanks, @nventuro for your suggestions.
I made the changes as per your suggestions. Please have a look.

[nventuro]

@princesinha19 to keep things organized, I merged this PR into #1704 (see this commit), while doing some slight modifications to a couple messages, so I'm closing this PR now in favor of the other one.

Thank you very much for your contribution!

[princesinha19]

Okay, no problem. @nventuro