DP-69 Continue SQS message processing loop if malformed message received

Fix a bug where receiving an SQS message that cannot be parsed as a Cheddar SimpleMessage would terminate the SQS message processing loop.

Signed-off-by: Steffan Westcott <steffan.westcott@clicktravel.com>
Better than returning null and checking for not-null would be to raise an exception and catch it.

One suggested name is MessageParsingException

After looking at the existing messaging exceptions, I think we could benefit from making a top-level abstract "MessagingException" which extends RuntimeException and then making all other other types of Message*Exception extend this.