Added 'NOT_FOUND' session failure cause.
Added 'NOT_FOUND' session failure cause.

Voiceworks/SIP.js,AaronZhangL/JsSIP,voxbone/JsSIP,joseph-onsip/SIP.js,onsip/SIP.js,dpocock/JsSIP,tqcenglish/JsSIP,DrumTechnologiesLtd/JsSIP,rusekr/JsSIP,markandrus/SIP.js,CodeYellowBV/JsSIP,josephfrazier/SIP.js,onsip/SIP.js,Escaux/SIP.js,ronlobo/SIP.js,vconsult/JsSIP