Handle missing README in create repos API (#23387)

Close #22934 

In `/user/repos` API (and other APIs related to creating repos), user
can specify a readme template for auto init. At present, if the
specified template does not exist, a `500` will be returned . This PR
improved the logic and will return a `400` instead of `500`.

routers/api/v1/repo/repo.go

@@ -231,6 +231,13 @@ func CreateUserRepo(ctx *context.APIContext, owner *user_model.User, opt api.Cre
 	if opt.AutoInit && opt.Readme == "" {
 		opt.Readme = "Default"
 	}
+
+	// If the readme template does not exist, a 400 will be returned.
+	if opt.AutoInit && len(opt.Readme) > 0 && !util.SliceContains(repo_module.Readmes, opt.Readme) {
+		ctx.Error(http.StatusBadRequest, "", fmt.Errorf("readme template does not exist, available templates: %v", repo_module.Readmes))
+		return
+	}
+
 	repo, err := repo_service.CreateRepository(ctx, ctx.Doer, owner, repo_module.CreateRepoOptions{
 		Name:          opt.Name,
 		Description:   opt.Description,
@@ -283,6 +290,8 @@ func Create(ctx *context.APIContext) {
 	// responses:
 	//   "201":
 	//     "$ref": "#/responses/Repository"
+	//   "400":
+	//     "$ref": "#/responses/error"
 	//   "409":
 	//     description: The repository with the same name already exists.
 	//   "422":
@@ -464,6 +473,8 @@ func CreateOrgRepo(ctx *context.APIContext) {
 	// responses:
 	//   "201":
 	//     "$ref": "#/responses/Repository"
+	//   "400":
+	//     "$ref": "#/responses/error"
 	//   "404":
 	//     "$ref": "#/responses/notFound"
 	//   "403":