Exceptions during automatic re-preparation are not converted to "nice" exceptions

To reproduce, do the following:

• Prepare a statement

• Drop the table that the statement acts on

• Execute the prepared statement

• A cassandra.protocol.InvalidRequestException will be raised instead of a cassandra.InvalidRequest

I believe the correct fix is to conditionally call response.to_exception() in ResponseFuture._execute_after_prepare(), similar to what we do in RF._set_result().