allow protocol generators to declare error aliases

[kuhe]

allow protocol implementations to use a set of string aliases when identifying errors

Codegen diff is in the unified error deserializer which relies on a string discriminator or identifier to route the error deserialization to the correct error class:

case "InvalidParameterCombinationException": // existing case
case "com.amazonaws.service#InvalidParameterCombinationException": // existing case
case "InvalidParameterCombination": // new: alias provided by AwsQueryCompat
  throw await de_InvalidParameterCombinationExceptionRes(parsedOutput, context);