RFE: check all invalidations before raising exception

[joshmoore]

Current validation implementations throw on the first error, rather than finding all validation errors and listing them. Instead, an exception with all the errors could be raised or alternatively a Validation object (or similar) could be returned.

Originally posted by @joshmoore in #157 (comment)

[sbesson]

See also #165 (comment) and #142.

When introducing schemas/constraints for validating, we should also review the associated validation framework & implementations and make sure the API has support for handling all validation errors, like Validator.iter_errors