Provide some more information for userland core dumps

Previously the log message indicated only "(core dumped)" if a core was
successfully created, or nothing if it was not. This provides
insufficient information to faciliate debugging. Dtrace is no help as
coredump() is static and we cannot find the return value via fbt.
Expand the log message to include error return value information.

Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D39942