Server-side callbacks are hiding errors / failing silently

[eric-burel]

Describe the bug

Callback behaviour is not predictible enough.

To Reproduce
For instance, I had forgottoen to import crypto in the hashPassword function of the user model.

The user creation callback was failing, but silently :(

Expected behavior

Fail explicitely, except if the callback is specifically marked as failing silently. More broadly, handle server-error correctly => they should return a 500 but should not fail the whole server.