Add test for issue 3489 and improve error messages.

api/filters/fieldspec/fieldspec.go

@@ -41,7 +41,7 @@ func (fltr Filter) Filter(obj *yaml.RNode) (*yaml.RNode, error) {
 	if err := fltr.filter(obj); err != nil {
 		s, _ := obj.String()
 		return nil, errors.WrapPrefixf(err,
-			"obj '%s' at path '%v'", s, fltr.FieldSpec.Path)
+			"considering field '%s' of object\n%v", fltr.FieldSpec.Path, s)
 	}
 	return obj, nil
 }

api/filters/fieldspec/fieldspec_test.go

@@ -176,8 +176,11 @@ kind: Bar
 a:
   b: a
 `,
-		error: "obj 'kind: Bar\na:\n  b: a\n' at path 'a/b/c': " +
-			"expected sequence or mapping node",
+		error: `considering field 'a/b/c' of object
+kind: Bar
+a:
+  b: a
+: expected sequence or mapping node`,
 		filter: fieldspec.Filter{
 			SetValue: filtersutil.SetScalar("e"),
 		},