[BUG] handles the program if the database is down ("service unavailable")

[JekxDevil]

Describe the bug
When we start the server and there is no database attached, or even when the latter goes down afterwards, the moment the back-end needs to provide a result using a query to ask to the database, the error throw by the fact that mongodb is down is not catch and make all the program crash.
Would be nice to catch it and redirect to a front-end dedicated page stating that the service is currently unavailable.
And redirect the user to the previous page (in M2, the login is the only one supported due to authorization) whenever the db connection becomes available again.

To Reproduce
Steps to reproduce the behavior:

1. Don't initialize the db service
2. Start the handshake server
3. See error

Expected behavior
Server crash. Error level: HIGH

Screenshots
none

Desktop:

• OS: Ubuntu 22.04 LTS
• Browser: Chromium
• Version: 108.0.5359.94 snap