bug: compound clients do insufficient compiler validation

[sxlijin]

User report via Slack connect: they have a fallback client where one of the clients doesn't actually exist, which causes prompt rendering to panic in unreachable!.

[sxlijin]

In the same vein, if the fallback has no clients, it also hits unreachable

[anish-palakurthi]

Also need to add syntax errors for unresolvable clients within fallbacks.