Refactor/error handling

[boyswan]

Keep RMRK errors intact rather than downcasting to Strings and wrapping in PSP34::Custom.

• Clear distinction between true PSP34 and RMRK errors
• Removes a lot of error boilerplate code
• Human readable logs in tests (Custom(String) prints as an encoded value)

@Maar-io made a good point about PSP34 potentially being a common interface between external contracts, so having our own error type produces are larger API surface area to cover. One way to manage this is to also provide a From<Error> for PSP34Error to allow easy conversions if necessary.

[Maar-io]

It is much more readable! Thanks for this PR
LGTM