Add error alerts for when Online Editor fails to open a file URL

Goals

When a file URL is passed using the `file` path param to the Online Editor, the user has to have a feedback if an error happens during the file fetch.

Acceptance Criteria

When a file URL is passed using the `file` path param, there are three possible outcomes while fetching the file:

1) File is fetched successfully and the editor opens with its content.

2) An error happens during file fetching, and no response is returned. In this case, an alert should be shown to the user:

3) An error response is returned when trying to fetch the file content. In this case, an alert like this should be shown to the user, showing the error code and description (if available):