Tell the user when we're retrying EC2 operations

  machine> creating EC2 instance (AMI ‘ami-96cb63e1’, type ‘c3.large’, region ‘eu-west-1’)...
  machine> got (possibly transient) EC2 error code ‘InvalidParameterValue’, retrying...

Of course, we probably shouldn't be retrying on InvalidParameterValue
in the first place, but now at least the user knows why things are
taking so long.