FC-73 Keep processing messages if 5xx error on SQS receive

SQS can sometimes give 5xx errors, though the SDK does retry if this occurs. When an 5xx error is returned, repeat the main loop anyway rather than stop processing.

Signed-off-by: Steffan Westcott <steffan.westcott@clicktravel.com>
After discussions it looks like ERROR is good (for now).