thread: Ignore errors when copying out during thr_exit()

It does not seem reasonable to return to userspace after calling
umtx_thread_exit().

This is in preparation for annotating copyin() and related functions
with __result_use_check.

Reviewed by: olce, kib
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D43143

(cherry picked from commit f450277f7a608f26624384e046c1987490c51296)