>Add a special failure echo when a character attempts to
>summon a mob that's "permanently" non-summonable. If the mob
>had just wandered into a nosummon room, or was on another
>continent, was just too high level for the summoner, or some
>other "transient" thing that would prevent summoning, then no
>special echo is given.

Why would you as a summoner know the reason for failure? You tried to summon them...and failed. It can literally be any of dozens of reasons, but the net effect is that you failed.

So long, and thanks for all the fish!

Depends on the role-play of the spell/commune I guess. I always envisioned the spell, at least, as somehow "pulling" the target to where you are. If you, the summoner, are "pulling", then it makes sense that you might be able to tell the difference between "almost moved" and "didn't budge an inch".