Restructure Errors by domain

[zupzup]

It might be nice to have error structures strictly by domain.

e.g. currently there is a bill service error and a general service error - each service should have their specific error type.

Also, ValidationError has all possible validation errors.

There should be a GeneralValidationError and specific ones like CompanyValidationError.

That way it would be easier for callers to know which errors can happen.