Deathnode should crash if AWS credentials are invalid

[alanbover]

When the application starts but it cannot get an IAM credentials, deathnode logs the next error infinitelly:

time="2017-08-29T16:12:22Z" level=debug msg="New check triggered"
time="2017-08-29T16:12:23Z" level=debug msg="Unable to find instances with DEATH_NODE_MARK_staging tag"
time="2017-08-29T16:12:23Z" level=error msg="NoCredentialProviders: no valid providers in chain. Deprecated. \n\tFor verbose messaging see aws.Config.CredentialsChainVerboseErrors"

Instead, deathnode should crash.