Validation errors should be able to indicate that a single error should be produced without collecting all of the details ahead of time

In general, validation is lazy -- if only one error is needed (in the sense that it is known that an instance is invalid), only one is produced.

There are however (somewhat unintentional) exceptions.

E.g.:

• anyOf and oneOf calculate the .context of the first error they yield by calculating all sub-errors that were generated. This is because ValidationError.context needs to be provided on __init__.
• dependentRequired, on the other hand, yields many errors, one for each missing required property (see also #119)

What's happening in these situations is that the validation function knows an error has occurred -- it wants to yield an error, but in order to do so, it needs to calculate the full details required so that the error is "complete".

We should instead have a way to indicate a validation error has occurred, but delay fully calculating its contents until/unless they are requested.

Start by tracing the validation paths for anyOf, oneOf, and dependentRequired, then inspect how ValidationError.init receives context. Done means a validation error can be yielded without collecting all sub-errors, while its full contents are calculated only when requested.