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
}
```

entc/gen/template/dialect/sql/update.tmpl

@@ -27,7 +27,7 @@ func ({{ $receiver }} *{{ $builder }}) sqlSave(ctx context.Context) ({{ $ret }}
 	{{- if $one }}
 		id, ok := {{ $mutation }}.{{ $.ID.MutationGet }}()
 		if !ok {
-			return {{ $zero }}, fmt.Errorf("missing {{ $.Name }}.ID for update")
+			return {{ $zero }}, &ValidationError{Name: "ID", err: fmt.Errorf("missing {{ $.Name }}.ID for update")}
 		}
 		_spec.Node.ID.Value = id
 	{{- else }}