[fty] deftagsum error not as clear as it could be

The following error is a bit misleading. It'd be clearer to say that venue-p doesn't have a definition.

```
HARD ACL2 ERROR in FTY::GET-TYPE-AND-FIX: Type VENUE-P doesn't have
an associated fixing function. Please provide that association using
FTY::DEFFIXTYPE.
```

The code to generate the error:

```
(include-book "std/top" :dir :system)
(include-book "centaur/fty/deftypes" :dir :system)

(std::defenum venue
(:stadium :convention-center :outdoors))

(fty::deftagsum sport
(:blurnball ((venue venue-p)))
(:golf ((venue venue-p)))
(:basketball ((venue venue-p)))
(:chess ())) ; no venue
```

I'm leaving unassigned -- I assume I can fix this pretty easily at some point.