Support for Custom URI Schemes in OAuth2 Redirect URIs (#37356)

Fix #34349

By the way, remove `(ctx *APIContext) HasAPIError() ` and `(ctx
*APIContext) GetErrMsg()` because they do nothing, the error handling
has been done in API's middeware

The existing OAuth2 tests were not quite right, refactored them together

templates/swagger/v1_json.tmpl

@@ -19354,6 +19354,9 @@
           "200": {
             "$ref": "#/responses/OAuth2Application"
           },
+          "400": {
+            "$ref": "#/responses/error"
+          },
           "404": {
             "$ref": "#/responses/notFound"
           }