Generic error handling needs to be reintegrated into spec

[marcoscaceres]

The following text was moved of the spec as part of the noLegacyStyle. It needs to be put back into each of the appropriate methods that it affects.

        In this final state ("disconnected"), the implementation MUST throw an
        <code>InvalidStateError</code> exception when any method call is
        attempted.

        "multiparty call" state: The call is locked in a <a>ConferenceCall</a>
        object and MUST NOT be managed any more using this object. While in
        this state, the implementation MUST throw an
        <code>InvalidStateError</code> exception when any method call is
        attempted.

[marcoscaceres]

Work in progress...

[marcoscaceres]

Relates to #47