Thanks to the generous help of MobesE46 on Github we now have better error handling in the MSDeployAllTheThings VSTS task.
https://github.com/rschiefer/MSDeployAllTheThings/commit/73685976787236e45d6258bcbc85959299a75f84
Several users had reported the task was not failing when the MSDeploy.exe execution had errors. I have now tested and verified the task fails when MSDeploy.exe returns an error.
I have updated the task in the VSTS marketplace and the new version is 0.0.8:
https://marketplace.visualstudio.com/items?itemName=rschiefer.MSDeployAllTheThings
Thanks to everyone involved both issue reporters and MobesE46 for the fix!