Introduce error util ErrInvalidCombination for invalid combination (#638

)

* Introduce error util ErrInvalidCombination for invalid combination

Sometimes valid value becomes invalid value by combination.

example 1. knative/serving#5382
example 2. following combination in `spec.traffic`.

```
  traffic:
  - latestRevision: true
    revisionName: hello-example-dk7nd
    percent: 100
```

But there are no error util for them, so we need to create
custom error like knative/serving@c1583f3
or `ErrInvalidValue`.

The custom error will make code complicated and `ErrInvalidValue` is
not debug friendly.

To solve it, this patch introduces an util func `ErrInvalidCombination`.

* Introduce ErrGeneric instead of ErrInvalidCombination

apis/field_error.go

@@ -312,20 +312,29 @@ func ErrDisallowedUpdateDeprecatedFields(fieldPaths ...string) *FieldError {
 }
 
 // ErrInvalidArrayValue constructs a FieldError for a repetetive `field`
-// at `index` that has received an invalid string value.
+// at `index` that has received an invalid value.
 func ErrInvalidArrayValue(value interface{}, field string, index int) *FieldError {
 	return ErrInvalidValue(value, CurrentField).ViaFieldIndex(field, index)
 }
 
 // ErrInvalidValue constructs a FieldError for a field that has received an
-// invalid string value.
+// invalid value.
 func ErrInvalidValue(value interface{}, fieldPath string) *FieldError {
 	return &FieldError{
 		Message: fmt.Sprintf("invalid value: %v", value),
 		Paths:   []string{fieldPath},
 	}
 }
 
+// ErrGeneric constructs a FieldError to allow for the different error strings for the
+// the different cases.
+func ErrGeneric(diagnostic string, fieldPaths ...string) *FieldError {
+	return &FieldError{
+		Message: diagnostic,
+		Paths:   fieldPaths,
+	}
+}
+
 // ErrMissingOneOf is a variadic helper method for constructing a FieldError for
 // not having at least one field in a mutually exclusive field group.
 func ErrMissingOneOf(fieldPaths ...string) *FieldError {

apis/field_error_test.go

@@ -158,6 +158,11 @@ Body.`,
 		err:      ErrInvalidValue(5*time.Second, "bar"),
 		prefixes: [][]string{{"baz"}},
 		want:     `invalid value: 5s: baz.bar`,
+	}, {
+		name:     "invalid value propagation",
+		err:      ErrGeneric("this is a generic error", "foo", "bar"),
+		prefixes: [][]string{{"baz"}},
+		want:     `this is a generic error: baz.bar, baz.foo`,
 	}, {
 		name:     "missing mutually exclusive fields",
 		err:      ErrMissingOneOf("foo", "bar"),