migration/cpr: Clean up error reporting in cpr_resave_fd()

migration/cpr: Clean up error reporting in cpr_resave_fd()

qapi/error.h advises:

 * Please don't error_setg(&error_fatal, ...), use error_report() and
 * exit(), because that's more obvious.

Do that, and replace exit() by g_assert_not_reached(), since this is
actually a programming error.

Cc: Steve Sistare <steven.sistare@oracle.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Steve Sistare <steven.sistare@oracle.com>
Message-ID: <20250923091000.3180122-5-armbru@redhat.com>
Reviewed-by: Akihiko Odaki <odaki@rsg.ci.i.u-tokyo.ac.jp>