ddb: make db_error reliably no-return

Most code assumes db_error does not return, but according to
kdb_reenter_silent, there may be cases where it could. Instead, panic
if kdb_reenter_silent returns and mark the routine as __dead2. This
addresses gcc warnings.

Reported by: GCC -Wmaybe-uninitialized
Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D45918