Adjust error handling of processing sub-process

Removes exception catching when collecting subprocess result which led
to the service silently go over failing file processing.

Now, the sub-process doesn't return any results if it failed. It is made
sure that an empty result is still returned if no images were present on
the file to process.

image_prediction/utils/process_wrapping.py

@@ -17,9 +17,6 @@ def wrap_in_process(fn):
         process = multiprocessing.Process(target=process_fn, args=args, kwargs=kwargs)
         process.start()
         process.join()
-        try:
-            return return_queue.pop(0)
-        except IndexError:
-            logger.warning("No results returned by subprocess.")
+        return return_queue.pop(0)
 
     return wrapped_fn