cue: don't wrap errors without message

Change-Id: I70f6b4fc12ba4136a0303d0863c41cbf2d64340f
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/2210
Reviewed-by: Marcel van Lohuizen <mpvl@google.com>
diff --git a/cmd/cue/cmd/testdata/partial/eval_conc.out b/cmd/cue/cmd/testdata/partial/eval_conc.out
index f2b417e..f26c68d 100644
--- a/cmd/cue/cmd/testdata/partial/eval_conc.out
+++ b/cmd/cue/cmd/testdata/partial/eval_conc.out
@@ -1,6 +1,6 @@
 sum:more than one element remaining (1 and 2):
     ./testdata/partial/partial.cue:4:6
-b.idx:: invalid non-ground value string (must be concrete int|string):
-    ./testdata/partial/partial.cue:7:7
+b.idx:invalid non-ground value string (must be concrete int|string):
+    ./testdata/partial/partial.cue:7:9
 b.str:incomplete value (string):
     ./testdata/partial/partial.cue:8:7
diff --git a/cmd/cue/cmd/testdata/partial/eval_expr.out b/cmd/cue/cmd/testdata/partial/eval_expr.out
index a4b4b79..0e251dd 100644
--- a/cmd/cue/cmd/testdata/partial/eval_expr.out
+++ b/cmd/cue/cmd/testdata/partial/eval_expr.out
@@ -1,5 +1,5 @@
 // b.a.b
 4
 // b.idx
-: invalid non-ground value string (must be concrete int|string):
-    ./testdata/partial/partial.cue:7:7
+invalid non-ground value string (must be concrete int|string):
+    ./testdata/partial/partial.cue:7:9
diff --git a/cue/evaluator.go b/cue/evaluator.go
index c277a6a..529e509 100644
--- a/cue/evaluator.go
+++ b/cue/evaluator.go
@@ -145,5 +145,7 @@
 			return b
 		}
 	}
-	return e.ctx.mkErr(v, codeIncomplete, e.bottom[0])
+	b := *e.bottom[0]
+	b.code = codeIncomplete
+	return &b
 }