frontend: simplify API and UI error handlers
    
    The error handler was too chatty and required a lot of unnecessary work
    to add a new exception to handle. The only thing that needs to be done
    now is adding a new `@app.errorhandler` decorator for `handle_exceptions`
    function. For cases where it's necessary, it still provides the flexibility
    to return a customized error page or error message for that exception.