RTCPeerConnection.createAnswer() - Web APIs
failurecallback an rtcpeerconnectionerrorcallback which will be passed a single domexception object explaining why the request to create an answer failed.

RTCPeerConnection.createOffer() - Web APIs
errorcallback an rtcpeerconnectionerrorcallback which will be passed a single domexception object explaining why the request to create an offer failed.

RTCPeerConnection.setLocalDescription() - Web APIs
errorcallback a function matching the signature rtcpeerconnectionerrorcallback which gets called if the description can't be set.

RTCPeerConnection.setRemoteDescription() - Web APIs
errorcallback a function matching the signautre rtcpeerconnectionerrorcallback which gets called if the description can't be set.