Improve validation errors

[eslavich]

The validation error is an unhelpful wall of text when a property fails validation in a complex schema. There ought to be something we can do to narrow the output to the relevant subschema.

[jdavies-st]

This is related to #689, and I suspect that if the validation error were cleaned up as indicated here, that would greatly improve the asdf schema pytest plugin.

[Cadair]

This is going to become a user-facing issue for us, as if we bump the schema of the files people try to load with old versions of our library they are going to get an incomprehensible error, when what it should be is "you need to update your install".