Better error message if the call to EC2 fails

Before this commit if the call to EC2 failed we get the following error
from the goamz/ec2 package:

    Request has expired. Timestamp date is 2013-09-09T15:16:57Z
    (RequestExpired)

Not very helpful. Unfortunately the goamz/ec2 package does not provide
more specifics as to what went wrong, so the best we can do is return
the following:

    EC2 API call failed; verify AWS credentials