Consider an API which collects validation errors and raises an exception group

[Julian]

Python 3.11 now supports exception groups for grouping together multiple (simultaneous, unrelated) exceptions.

This seems to suit us -- right now we have:

• Validator.iter_errors, aimed at returning (not raising) all errors
• Validator.is_valid, aimed at returning no errors and indicating via a boolean whether things are OK
• Validator.validate, aimed at raising 1 single exception (the first encountered one)

Another API where we raise all exceptions iter_errors would have returned seems like it may fit the picture.

Reviewing the notes API to see if we should make any use of it is likely also wise.