Structured error from Constraint on invalid input

[mih]

Right now, a constraint will merely error on invalid input, typically with a ValueError.

For more complex constraints (nested, alternatives), the reason for an "invalid" response can be difficult to decode. Moreover, it is impossible to associate an error with a particular input component, in case of validating a structured data input.

An exception needs to be enhanced with enough information to pinpoint the reason for a validation failure -- enough to be able to annotate the respective component of an input form with a targeted error message.

CC @jsheunis

[mih]

Pydantic's ValidationError readily provides this kind of information.