Update messaging to represent the real issue when "converging" fails

[athompson-r7]

In this particular case the user's AWS credentials expired, so they were unable to continue polling the events for the stack that was being created. This is fine as Cloudformation is doing the right thing in the background, but the error presented to the user is confusing suggesting that something failed and that they need to do something about it.

We need to differentiate between failures in fetching events and converging.

create_in_progress  StagingCDNBucketPolicy: (AWS::S3::BucketPolicy/)
       error  Error converging stack s3-buckets
* The security token included in the request is expired