Fix bug related with pagination

There was a bug which avoided to render the errorCode with "details=on". Render methods have been modified and also some unit tests that were broken with the modifications.
The same applies here too.

[ This time I 'explain' what I mean in a different way ]

OMA 5.3.2.2: 

    The response SHALL contain exactly one of the following:
    - ContextRegistrationResponseList
    - ErrorCode

'exactly one' is key here