[FIX] base: clearer message in case of conflict for no_gap sequences

The raw error message from psycopg2 wasn't clear for users and led to 
multiple support tickets.

In case of such "concurrency" errors, the user should retry later, it's 
not a "real" bug.

closes odoo/odoo#57810

Signed-off-by: Victor Feyens (vfe) <vfe@odoo.com>