fix: Improve error message returned by Subsume

Previously when Subsume determined that a field did not subsume
a previous field, the error returned was:

```
field x not present in <value>
```

which is simply not true. Now a more useful error is returned in this
scenario:

```
31 | 33 does not subsume 32:
    ./schema.cue:2:25
    ./schema.cue:7:25
```

Resolves: cue-lang#3861

Signed-off-by: Graham Dennis <[email protected]>

Author: GrahamDennis

Diff for: internal/core/subsume/subsume.go

@@ -122,7 +122,7 @@ func (s *subsumer) getError() (err errors.Error) {
 		if s.missing != 0 {
 			s.errf("missing field %q", s.missing.SelectorString(c))
 		} else if b, ok := unifyValue(c, s.gt, s.lt).(*adt.Bottom); !ok {
-			s.errf("value not an instance")
+			s.errf("%v does not subsume %v", s.gt, s.lt)
 		} else {
 			s.errs = errors.Append(s.errs, b.Err)
 		}

Diff for: internal/core/subsume/vertex.go

@@ -170,11 +170,19 @@ func (s *subsumer) vertices(x, y *adt.Vertex) bool {
 			continue
 		}
 
-		s.missing = f
-		s.gt = a
-		s.lt = y
+		if b == nil {
+			s.missing = f
+			s.gt = a
+			s.lt = y
+
+			s.errf("field %v not present in %v", f, y)
+		} else {
+			s.gt = a
+			s.lt = b
+
+			s.errf("%v in %v does not subsume %v in %v", a, x, b, y)
+		}
 
-		s.errf("field %v not present in %v", f, y)
 		return false
 	}
 
@@ -373,11 +381,18 @@ func (s *subsumer) verticesDev(x, y *adt.Vertex) bool {
 			continue
 		}
 
-		s.missing = f
-		s.gt = a
-		s.lt = y
+		if b == nil {
+			s.missing = f
+			s.gt = a
+			s.lt = y
+
+			s.errf("field %v not present in %v", f, y)
+		} else {
+			s.gt = a
+			s.lt = b
 
-		s.errf("field %v not present in %v", f, y)
+			s.errf("%v does not subsume %v", a, b)
+		}
 		return false
 	}