Fix filename check, add helpful note if AWS creds are wrong
Fix filename check, add helpful note if AWS creds are wrong

tobilg/dcos-commons,comptelfwd/dcos-commons,comptelfwd/dcos-commons,adragomir/dcos-commons,mesosphere/dcos-commons,adragomir/dcos-commons,tobilg/dcos-commons,mesosphere/dcos-commons,adragomir/dcos-commons,comptelfwd/dcos-commons,comptelfwd/dcos-commons,vishnu2kmohan/dcos-commons,mesosphere/dcos-commons,vishnu2kmohan/dcos-commons,comptelfwd/dcos-commons,vishnu2kmohan/dcos-commons,comptelfwd/dcos-commons,mesosphere/dcos-commons,adragomir/dcos-commons,adragomir/dcos-commons,vishnu2kmohan/dcos-commons,tobilg/dcos-commons,vishnu2kmohan/dcos-commons,mesosphere/dcos-commons,tobilg/dcos-commons,tobilg/dcos-commons