Thread::quit was created before the pthread_create_helper in pthread.cpp
that automagically calls pthread_exit from all pthreads after the user's
thread function exits. It is unused, and unecessary now.
Cleanup some logging, and make join return a Result<T, ThreadError>.
This also adds a new type, LibThread::ThreadError as an
AK::DistinctNumeric. Hopefully, this will make it possible to have a
Result<int, ThreadError> and have it compile? It also makes it clear
that the int there is an error at the call site.
By default, the T on join is void, meaning the caller doesn't care about
the return value from the thread.
As Result is a [[nodiscard]] type, also change the current caller of
join to explicitly ignore it.
Move the logging out of join as well, as it's the user's
responsibility whether to log or not.
