spapr_events: Sure we may ignore migrate_add_blocker() failure?

[Markus Armbruster <armbru@redhat.com>]

Commit 2500fb423a "migration: Include migration support for machine
check handling" adds this:

    ret = migrate_add_blocker(spapr->fwnmi_migration_blocker, &local_err);
    if (ret == -EBUSY) {
        /*
         * We don't want to abort so we let the migration to continue.
         * In a rare case, the machine check handler will run on the target.
         * Though this is not preferable, it is better than aborting
         * the migration or killing the VM.
         */
        warn_report("Received a fwnmi while migration was in progress");
    }

migrate_add_blocker() can fail in two ways:

1. -EBUSY: migration is already in progress

   Ignoring this one is clearly intentional.  The comment explains why.
   I'm taking it at face value (I'm a spapr ignoramus).  Aside: I doubt
   the warning is going to help users.

2. -EACCES: we're running with -only-migratable

   Why may we ignore -only-migratable here?

By the way, we leak @local_err on failure.  I'll post a patch, but I'd
like my question answered first.