entc/gen: introduce validation errors (#547)

This introduces the concept of validation errors, where we have a high
level validation error which wraps a more detailed error message.

The higher level `ValidationError` is set up in the generated files, much
like the `NotFoundError` and `ConstraintError` and is accompanied by an
`IsValidationError` check method. Thus, it can be used as follows:

```go
t, err := tx.Team.Create().SetName(input.Name).Save(ctx)
if ent.IsValidationError(err) {
        // handle validation error response
}
```

examples/m2m2types/ent/ent.go

@@ -100,6 +100,31 @@ func Sum(field string) AggregateFunc {
 	}
 }
 
+// ValidationError returns when validating a field fails.
+type ValidationError struct {
+	Name string // Field or edge name.
+	err  error
+}
+
+// Error implements the error interface.
+func (e *ValidationError) Error() string {
+	return e.err.Error()
+}
+
+// Unwrap implements the errors.Wrapper interface.
+func (e *ValidationError) Unwrap() error {
+	return e.err
+}
+
+// IsValidationError returns a boolean indicating whether the error is a validaton error.
+func IsValidationError(err error) bool {
+	if err == nil {
+		return false
+	}
+	var e *ValidationError
+	return errors.As(err, &e)
+}
+
 // NotFoundError returns when trying to fetch a specific entity and it was not found in the database.
 type NotFoundError struct {
 	label string

examples/m2m2types/ent/group_create.go

@@ -53,7 +53,7 @@ func (gc *GroupCreate) Mutation() *GroupMutation {
 // Save creates the Group in the database.
 func (gc *GroupCreate) Save(ctx context.Context) (*Group, error) {
 	if _, ok := gc.mutation.Name(); !ok {
-		return nil, errors.New("ent: missing required field \"name\"")
+		return nil, &ValidationError{Name: "name", err: errors.New("ent: missing required field \"name\"")}
 	}
 	var (
 		err  error

examples/m2m2types/ent/group_update.go

@@ -312,7 +312,7 @@ func (guo *GroupUpdateOne) sqlSave(ctx context.Context) (gr *Group, err error) {
 	}
 	id, ok := guo.mutation.ID()
 	if !ok {
-		return nil, fmt.Errorf("missing Group.ID for update")
+		return nil, &ValidationError{Name: "ID", err: fmt.Errorf("missing Group.ID for update")}
 	}
 	_spec.Node.ID.Value = id
 	if value, ok := guo.mutation.Name(); ok {

examples/m2m2types/ent/user_create.go

@@ -59,10 +59,10 @@ func (uc *UserCreate) Mutation() *UserMutation {
 // Save creates the User in the database.
 func (uc *UserCreate) Save(ctx context.Context) (*User, error) {
 	if _, ok := uc.mutation.Age(); !ok {
-		return nil, errors.New("ent: missing required field \"age\"")
+		return nil, &ValidationError{Name: "age", err: errors.New("ent: missing required field \"age\"")}
 	}
 	if _, ok := uc.mutation.Name(); !ok {
-		return nil, errors.New("ent: missing required field \"name\"")
+		return nil, &ValidationError{Name: "name", err: errors.New("ent: missing required field \"name\"")}
 	}
 	var (
 		err  error

examples/m2m2types/ent/user_update.go

@@ -352,7 +352,7 @@ func (uuo *UserUpdateOne) sqlSave(ctx context.Context) (u *User, err error) {
 	}
 	id, ok := uuo.mutation.ID()
 	if !ok {
-		return nil, fmt.Errorf("missing User.ID for update")
+		return nil, &ValidationError{Name: "ID", err: fmt.Errorf("missing User.ID for update")}
 	}
 	_spec.Node.ID.Value = id
 	if value, ok := uuo.mutation.Age(); ok {