Rudimentary error handling for non-main threads

Rudimentary error handling for non-main threads

* src/thread.c (last_thread_error): New static variable.
(syms_of_threads): Staticpro it.
(record_thread_error, Fthread_last_error): New functions.
(syms_of_threads): Defsubr Fthread_last_error.

* doc/lispref/threads.texi (Basic Thread Functions): Document
thread-last-error.

* test/src/thread-tests.el (thread-errors, thread-signal-early)
(threads-condvar-wait): Test the values returned by
thread-last-error.