Add SmartQueue with get() that raises execptions in queue

To keep read IO fully utilized (especially during imports from card readers), we read in a separate thread. This is all fine and good unless an exception occurs in the reader thread... the thread sort of eats the exception, and we don't have a well-defined condition in the main thread. It's difficult to debug and confusing to the programmer.

So instead of queue.Queue, we're going to use a new SmartQueue class something like this:

class SmartQueue(Queue):
    def get(self, block=True, timeout=None):
        item = super().get(block, timeout)
        if isinstance(item, Exception):
            raise item
        return item

And then you'll update the two worker functions reader() and batch_reader() so they wrap everything in a try/except like this:

def reader(src_fp, queue):
    try:
        # Do stuff...
    except Exception as e:
        queue.put(e)

Which is all pretty easy... but then you also have to write the test for SmartQueue.get(), and update the tests for reader() and batch_reader() to make sure they handle exception scenarios correctly.