Bug fixes regarding ngsi10 subscriptions

### Description
Two bug fixes:
  o Fixed rendering of NGSI10 SubscribeContextResponse.
  o Also made sure subscriptionId is included in SubscribeError for UpdateContextSubscription.

### Tests
All affected unit and harness tests modified accordingly.

### Coverage
I haven't even looked at the coverage.
My next PR will be about json payload for SubscribeContext and the coverage will be set at 100%.