add "InvalidIdentifier" as a subclass Error for ServiceError
add "InvalidIdentifier" as a subclass Error for ServiceError

- when creating a new ServiceError, if the message contains the strings
  "Invalid ASIN identifier (x) for marketplace (y)", you'll receive back
  at the client end, a proper error giving clues as to what might be wrong

ericblade/mws-advanced,ericblade/mws-advanced