Implement error checking for pthreads calls in thread-safe mode. They really

Implement error checking for pthreads calls in thread-safe mode. They really
should always succeed, but in the likely event of a failure we would
previously fall through *without locking* - the new code will exit(1).

Printing the error message on stderr will not work for all applications, but
it's better than nothing at all - and our API doesn't provide a way to return
the error to the caller.