Support custom errors

[greyblake]

Context

Some orgs and people are not quite happy with auto-generated error types, e.g. it's not possible to customize them, inject extra details, etc.

Requirement

Provide a way for users to:

• Define and provide error type they want to use
• Inject their custom validation function that would return Result<(), CustomError>, where CustomError is a user defined error.

Technically it's possible, but the syntax for this needs to be decided.

Consideration

Having this may clash with "unified error": https://github.com/greyblake/nutype/issues/75, so requires careful thinking.