Error handling about erc721a

[Vectorized]

@cygaar

For the next version (after upgradeable), I plan to add Natspec for all custom errors and move them into the class level instead of the file level.

Tests won't be affected.

This will give meaningful error messages in logging where supported (e.g. Remix console).

[kyokosdream]

When solc compiles the contract, comments are not included in the compiled bytecode. Comments do not add any gas usage. I don't know of any popular languages that would compile comments.

The errors are self-descriptive and I'm not sure why we would need comments added to them. It might help beginner users understand the ERC721A contract just slightly better but seems rather unnecessary.

[Vectorized]

May probably add NatSpec to the custom errors, someday in the far future, like: https://twitter.com/pcaversaccio/status/1505479350363332611?s=21

For now, we’ll just leave it as it is.

The names are simply the original revert messages in camelCase, so there is no loss of semantic information. As custom errors drastically reduces deployment gas costs, we merged it ASAP.