Apply project best practices to switches over enum values.
In the wake of
1f3a02173, assorted buildfarm members were warning about
"control reaches end of non-void function" or the like. Do what we've
done elsewhere: in place of a "default" switch case that will prevent
the compiler from warning about unhandled enum values, put a catchall
elog() after the switch. And return a dummy value to satisfy compilers
that don't know elog() doesn't return.