ExecNIOSource blocks indefinitely if an exception is encountered

In the ExecNIOSource, any exception that occurs in one of the two reader threads causes the source to stop transferring data[1] but continue to run. This, in turn, leaves the source in a state that requires human intervention (i.e. the configuration needs to be refreshed or the node restarted).

[1] This is because the reader thread for one of stderr or stdout will exit but the next() call which is reading from a blocking queue won't know a thread died.