* add error_union value tag.
* add analyzeIsErr
* add Value.isError
* add TZIR wrap_errunion_payload and wrap_errunion_err for
  wrapping from T -> E!T and E -> E!T
* add anlyzeInstUnwrapErrCode and analyzeInstUnwrapErr
* add analyzeInstEnsureErrPayloadVoid:
* add wrapErrorUnion
* add comptime error comparison for tests
* tests!