Check whether worker threads spawn successfully

[LouisJenkinsCS]

Right now, in Montage, there is an assumption by the ParallelLaunch code that worker threads will always spawn successfully, i.e. there is no error checking or handling.

Montage/src/ParallelLaunch.cpp

Line 218 in 44d55f9

pthread_create (&threads[i], &attr, thread_main, &ctcs[i]);

I'm running into an issue where for some reason, I cannot seem to spawn any threads, but I assumed it did because no error was reported by Montage. Note that if the workers fail, after calling init, the main thread will just deadlock, waiting at the barrier for its non-existent brethren.