AWS Uninstall hangs on DELETE_FAILED

[nzoschke]

If the stack delete fails (see #66), the uninstall script doesn't quit. It should exit with a non-zero status code.

...
DELETE_COMPLETE       InstancesSecurity             AWS::EC2::SecurityGroup
DELETE_COMPLETE       InstancesRole                 AWS::IAM::Role
DELETE_IN_PROGRESS    Network                       AWS::CloudFormation::Stack
DELETE_COMPLETE       Volumes                       AWS::EFS::FileSystem
DELETE_COMPLETE       Network                       AWS::CloudFormation::Stack
DELETE_FAILED         test                          AWS::CloudFormation::Stack