internal: phase out DecorateError in favor of errors.Wrap

The former seems to have been a thing in the old evaluator code
deleted in 2020, but a single call remained in the subsume package.

All that a decorated error did was augment an errors.Error value
with a Go error so that errors.Is would work with either.
errors.Wrap gives us the same property, and is more consistent
with the rest of the codebase nowadays.

Signed-off-by: Daniel Martí <mvdan@mvdan.cc>
Change-Id: I5a3acf0381c83e0e36c18708cfb586ceccc7e2f2
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1200973
Reviewed-by: Roger Peppe <rogpeppe@gmail.com>
TryBot-Result: CUEcueckoo <cueckoo@cuelang.org>
Unity-Result: CUE porcuepine <cue.porcuepine@gmail.com>

internal/core/subsume/subsume.go

@@ -138,7 +138,7 @@ func (s *subsumer) getError() (err errors.Error) {
 	}
 	err = s.errs
 	if s.inexact {
-		err = internal.DecorateError(internal.ErrInexact, err)
+		err = errors.Wrap(err, internal.ErrInexact)
 	}
 	return err
 }

internal/internal.go

@@ -430,19 +430,3 @@ func GenPath(root string) string {
 }
 
 var ErrInexact = errors.New("inexact subsumption")
-
-func DecorateError(info error, err errors.Error) errors.Error {
-	return &decorated{cueError: err, info: info}
-}
-
-type cueError = errors.Error
-
-type decorated struct {
-	cueError
-
-	info error
-}
-
-func (e *decorated) Is(err error) bool {
-	return errors.Is(e.info, err) || errors.Is(e.cueError, err)
-}