Improve error messages when `oneOf` fails because of constraint-driven type-specific keywords

[jviotti]

Consider this schema:

{
  "$schema": "https://json-schema.org/draft/2020-12/schema",
  "oneOf": [
    { "required": [ "foo" ] },
    { "required": [ "bar" ] },
    { "required": [ "baz" ] }
  ]
}

And this instance:

[ { "foo": 1 } ]

Trace validation gives us this:

$ jsonschema validate schema.json instance.json --trace
-> (push) "/oneOf" (LogicalXor)
   at ""
   at keyword location "file:///Users/jviotti/Projects/playground/schema.json#/oneOf"
   at vocabulary "https://json-schema.org/draft/2020-12/vocab/applicator"

<- (fail) "/oneOf" (LogicalXor)
   at ""
   at keyword location "file:///Users/jviotti/Projects/playground/instance.json#/oneOf"
   at vocabulary "https://json-schema.org/draft/2020-12/vocab/applicator"

We have no indication on which branches match or didn't match the oneOf at all. Also no indication that required worked because of the constraint-driven nature of JSON Schema.

It would have been nice to get something like this instead:

$ jsonschema validate schema.json instance.json --trace
-> (push) "/oneOf" (LogicalXor)
   at ""
   at keyword location "file:///Users/jviotti/Projects/playground/schema.json#/oneOf"
   at vocabulary "https://json-schema.org/draft/2020-12/vocab/applicator"

-> (push) "/oneOf/0/required"
   at ""

-> (pass) "/oneOf/0/required"
   at ""
   warning: Passed because of a type mismatch

-> (push) "/oneOf/1/required"
   at ""

-> (pass) "/oneOf/1/required"
   at ""
   warning: Passed because of a type mismatch

-> (push) "/oneOf/2/required"
   at ""

-> (pass) "/oneOf/2/required"
   at ""
   warning: Passed because of a type mismatch

<- (fail) "/oneOf" (LogicalXor)
   at ""
   at keyword location "file:///Users/jviotti/Projects/playground/instance.json#/oneOf"
   at vocabulary "https://json-schema.org/draft/2020-12/vocab/applicator"

[jviotti]

So the "problem" is on Blaze. The required keywords compile to AssertionDefines, but that instruction (like many others), have a pre-condition for its corresponding type, and if the pre-condition doesn't hold (is_object() in the case), the Pre/Post callbacks are never emitted.

Maybe what we should do is update evaluator_complete.h to fire a new Skip callback in these cases?