Need more expressive error messages when credentials are wrong or missing

[amirfarhat]

When trying to eliminate the use of environment variables, instead relying on the AWS service, this error message was displayed. The problem was that the environment did not have access to the iap-secrets variables. A better error message could simply mention it has no access to iap-secrets. This is the error:

Missing master_SLACK_TOKEN
Makefile.windows:78: recipe for target 'ecs_stop' failed
make: *** [ecs_stop] Error 1

[davidchaiken]

similarly, need a more expressive error message for this line when credentials are not correct:

docker-compose -f cmds.yml run aws ecr get-login \
                    --region us-east-1 --no-include-email > limbo-tmp