Clean up pthread_create() error handling

Clean up pthread_create() error handling

Normally pthread_create() rarely fails. But with new pthreads wrapper,
pthread_create() will return ENOSYS on a system without thread support.

Threaded code _is_ protected by HAVE_THREADS and pthread_create()
should never run in the first place. But the situation could change in
the future and bugs may sneak in. Make sure that all pthread_create()
reports the error cause.

While at there, mark these strings for translation if they aren't.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>