[webrtc-insertable-streams] Handle SFrame error case when encrypting/decrypting (#58)

youennf has just created a new issue for https://github.com/w3c/webrtc-insertable-streams:

== Handle SFrame error case when encrypting/decrypting ==
We need to handle some potential error cases:
- no key set on encryption side
- no key for the given ID on decryption side
- Decryption error


Please view or discuss this issue at https://github.com/w3c/webrtc-insertable-streams/issues/58 using your GitHub account


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