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: #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)
 		}