Apply project best practices to switches over enum values.

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.