* Error handling in ERC20 and ERC721

* Added message string for require.

* Fixed solhint errors.

* Updated PR as per issue #1709

* changes as per #1709 and openzeppelin forum.

* Changes in require statement

* Changes in require statement

* build pipeline fix

* Changes as per @nventuro's comment.

* Update revert reason strings.

* Fianal update of revert reason strings.

* WIP: Updating reason strings in test cases

* WIP: Added changes to ERC20 and ERC721

* Fixes linting errors in *.tes.js files

* Achieved 100% code coverage

* Updated the test cases with shouldFail.reverting.withMessage()

* Fix package-lock.

* address review comments

* fix linter issues

* fix remaining revert reasons