net.box: fix `on_disconnect` inconsistency with documentation

According to the documentation [1]:
> If the trigger function causes an error, the error is logged but
otherwise is ignored.

However, currently, the `on_disconnect` trigger behaves the same way as the
`on_connect` trigger, i.e., the connection is terminated and its state
changes to 'error'. Let's fix this inconsistency and log errors from the
`on_disconnect` trigger, but otherwise ignore them.

Closes #9677
Closes #9797

NO_DOC=<bugfix>

1. https://www.tarantool.io/en/doc/latest/reference/reference_lua/net_box/#lua-function.conn.on_disconnect