Up to now the /me/changePassword endpoint return a 500 Status when
issue a password change with the old password set to the wrong password.
This changes the code to return 400 (Bad Request) with an additional
message that the old password is wrong. This does not seem to weaken the
security of /me/changePassword (i.e. for allowing easier brute-force
attacks) as the endpoint is only available to already authenticated
users (and only for changing their own passwords)
See #4480
