Add missing error checking for kernel_port_{add,remove}().  Both can fail
for reasons yet unknown; don't make it increment cumulated_error as a kind
of temporary workaround.

MFC after:	1 month
Sponsored by:	The FreeBSD Foundation


git-svn-id: svn+ssh://svn.freebsd.org/base/head@274791 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f