fix: validate usernames on registration

pkg/routes/validation.go

@@ -17,6 +17,8 @@
 package routes
 
 import (
+	"net/http"
+
 	"code.vikunja.io/api/pkg/models"
 
 	"code.vikunja.io/web"
@@ -41,15 +43,14 @@ func (cv *CustomValidator) Validate(i interface{}) error {
 			errs = append(errs, field+": "+e)
 		}
 
-		httperr := models.ValidationHTTPError{
+		return models.ValidationHTTPError{
 			HTTPError: web.HTTPError{
-				Code:    models.ErrCodeInvalidData,
-				Message: "Invalid Data",
+				HTTPCode: http.StatusPreconditionFailed,
+				Code:     models.ErrCodeInvalidData,
+				Message:  "Invalid Data",
 			},
 			InvalidFields: errs,
 		}
-
-		return httperr
 	}
 	return nil
 }