UA#onTransportError: don't try next transport if UA is closed
UA#onTransportError: don't try next transport if UA is closed

addresses #63

joseph-onsip/SIP.js,josephfrazier/SIP.js,onsip/SIP.js,ronlobo/SIP.js,Escaux/SIP.js,markandrus/SIP.js,onsip/SIP.js