Add status message to BW test Gitlab and Mattermost feedback

Goes with Moore!2603 and PRConfig!342.

One problem with the BW tests is that, to get the handler to run, they must force an exit code of 0, so you can't automatically tell if all sub-jobs within it are successful or not. In the PRConfig MR we now start tracking the error codes of all the subjobs, and this gets passed now to the handler via the message.txt that it reads. This MR now translates that error code into a message that will be added to the Gitlab and Mattermost feedback.