Fix generated SDK errors to use pointer receivers

[jasdel]

Fixes the generated SDK API errors to use pointer function receivers instead of value. This fixes potential confusion writing code and not casting to the correct type. The SDK will always return the API error as a pointer, not value.

Code that did type assertions from the operation's returned error to the value type would never be satisfied. Leading to errors being missed. Changing the function receiver to a pointer prevents this error. Highlighting it in code bases.

For example the following type assertion handling a service response would never work because the cloudwatchlogs.DataAlreadyAcceptedException error is returned as a pointer, not value.

resp, err := client.SomeOperationCall(params)
if err != nil {
     switch tv := err.(type)  {
     case cloudwatchlogs.DataAlreadyAcceptedException:
          log.Printf("got DataAlreadyAcceptedException error, %s", tv.String()
     default:
          log.Printf("unknown error %T, %v", err, err)
     }
}

The code would need to be written with the type assertion on the pointer value, which is what the SDK will return.

resp, err := client.SomeOperationCall(params)
if err != nil {
     switch tv := err.(type)  {
     case *cloudwatchlogs.DataAlreadyAcceptedException:
          log.Printf("got DataAlreadyAcceptedException error, %s", tv.String()
     default:
          log.Printf("unknown error %T, %v", err, err)
     }
}