Added error handling logic for ERROR frames - Dogan

[tepafoo]

Hello coders,

For your kind review and pull.

Commit message:

"Added logic to register error callbacks on connect() and de-register on disconnect(). All registered error callbacks get called on reception of ERROR frames. Also removed unnecessary callback in subscribe() method signature."

Cheers

Dogan

[easternbloc]

Looks good to me. I'll address the mocking issues at some point as I'm not overly fond of those. Cheers.

[tepafoo]

:)