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/admin/repo.go

@@ -32,6 +32,8 @@ func CreateRepo(ctx *context.APIContext) {
 	// responses:
 	//   "201":
 	//     "$ref": "#/responses/Repository"
+	//   "400":
+	//     "$ref": "#/responses/error"
 	//   "403":
 	//     "$ref": "#/responses/forbidden"
 	//   "404":