feat: Adding basic error type support for output units.

[CheesyBoy123]

Motivation:

As described in #132 we want to add support for OutputErrorTypes. Whenever we have an error there should be some kind of error type associated. For instance if the field is required but is missing we should return back the error type of MISSING_VALUE.

For recursive error checks like $ref, $anyOf, $not etc, the error type is taken from the recursive OutputUnit.
For primitive checks like array length checks, format validation, required checks etc I have added what I believe the correct Error Type should be.

Conformance:

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