Display stack fetch request errors

[adammw]

Currently, a stack diff/update can fail with cryptic "Failed to locate requested stack" without telling you why it could not be found. This will print the error in the UI when it is not a simple not found error (e.g. permission denied etc.)

Example output:

[ERROR]: Forbidden - AccessDenied: User: arn:aws:sts::12345:assumed-role/myuser is not authorized to perform: sts:AssumeRole on resource: arn:aws:iam::12345:role/myrole
[FATAL]: Failed to locate requested stack: my-stack-name