How to use the safety.errors.DatabaseFetchError function in safety

if output:
            with open(output, 'w+') as output_file:
                output_file.write(output_report)
        else:
            click.secho(output_report, nl=False if bare and not vulns else True)
        sys.exit(-1 if vulns else 0)
    except InvalidKeyError:
        click.secho("Your API Key '{key}' is invalid. See {link}".format(
            key=key, link='https://goo.gl/O7Y1rS'),
            fg="red",
            file=sys.stderr)
        sys.exit(-1)
    except DatabaseFileNotFoundError:
        click.secho("Unable to load vulnerability database from {db}".format(db=db), fg="red", file=sys.stderr)
        sys.exit(-1)
    except DatabaseFetchError:
        click.secho("Unable to load vulnerability database", fg="red", file=sys.stderr)
        sys.exit(-1)

if output:
            with open(output, 'w+') as output_file:
                output_file.write(output_report)
        else:
            click.secho(output_report, nl=False if bare and not vulns else True)
        sys.exit(-1 if vulns else 0)
    except InvalidKeyError:
        click.secho("Your API Key '{key}' is invalid. See {link}".format(
            key=key, link='https://goo.gl/O7Y1rS'),
            fg="red",
            file=sys.stderr)
        sys.exit(-1)
    except DatabaseFileNotFoundError:
        click.secho("Unable to load vulnerability database from {db}".format(db=db), fg="red", file=sys.stderr)
        sys.exit(-1)
    except DatabaseFetchError:
        click.secho("Unable to load vulnerability database", fg="red", file=sys.stderr)
        sys.exit(-1)

if db:
        mirrors = [db]
    else:
        mirrors = API_MIRRORS if key else OPEN_MIRRORS

    db_name = "insecure_full.json" if full else "insecure.json"
    for mirror in mirrors:
        # mirror can either be a local path or a URL
        if mirror.startswith("http://") or mirror.startswith("https://"):
            data = fetch_database_url(mirror, db_name=db_name, key=key, cached=cached, proxy=proxy)
        else:
            data = fetch_database_file(mirror, db_name=db_name)
        if data:
            return data
    raise DatabaseFetchError()

class DatabaseFetchError(Exception):
    pass


class DatabaseFileNotFoundError(DatabaseFetchError):
    pass


class InvalidKeyError(DatabaseFetchError):
    pass

class DatabaseFetchError(Exception):
    pass


class DatabaseFileNotFoundError(DatabaseFetchError):
    pass


class InvalidKeyError(DatabaseFetchError):
    pass