Check g:weirdplugin.name in vroom, for clarity and nicer failures

Mainly I want this to demonstrate the remaining issue in google/vroom#76. The existing failure message just says `E121: Undefined variable: g:installed_weird_p_l_u_g_i_n`, but after this change it explains:
```
Unexpected message:
weird¬pâ<80>¦l✓u↓g⏎iâ<80>½n

Expected message not received:
"weird¬p…l✓u↓g⏎i‽n" (verbatim mode)
```