internal/core/runtime: do not hide positions in interpreter errors

Currently the error wrapping logic in `extern.go` will hide any
positions in the error returned by the interpreter, which ends up hiding
useful position information returned by the embed implementation.

Fix that by using `errors.Wrap` instead of `errors.New`.

Fixes #3320.

Signed-off-by: Roger Peppe <[email protected]>
Change-Id: I9e50e90456528164b64e74af646a66faf0c90fdf
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1198350
Reviewed-by: Daniel Martí <[email protected]>
Unity-Result: CUE porcuepine <[email protected]>
TryBot-Result: CUEcueckoo <[email protected]>

Author: rogpeppe

cmd/cue/cmd/testdata/script/embed_file_err.txtar

@@ -1,5 +1,5 @@
 # Test what happens when there is an error an embedded file.
-# TODO the error should mention the name of the file where the error is.
+# The error should mention the name of the file where the error is.
 
 env CUE_EXPERIMENT=embed
 ! exec cue vet
@@ -12,6 +12,7 @@ cmp stderr want-stderr
 -- want-stderr --
 @embed: invalid string:
     ./x.cue:5:6
+    schema.json:10:7
 -- cue.mod/module.cue --
 module: "test.example"
 language: version: "v0.9.2"

internal/core/runtime/extern.go

@@ -332,7 +332,7 @@ func (d *externDecorator) processADTNode(n adt.Node, scope *adt.Vertex) bool {
 
 	b, err := c.Compile(name, scope, &attr)
 	if err != nil {
-		err = errors.Newf(info.attr.Pos(), "@%s: %v", info.extern, err)
+		err = errors.Wrap(errors.Newf(info.attr.Pos(), "@%s", info.extern), err)
 		d.errs = errors.Append(d.errs, err)
 		return true
 	}