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>
ERROR is the more appropriate level, as it higher in priority than WARN and so is less likely to be filtered away. Also, according to the documentation, WARN is intended for "potentially harmful situations", whereas ERROR is for "error events that might still allow the application to continue running". ERROR is one level below FATAL.