Don't blindly hangup on a call error.

Not all errors means we want to send a hangup (in fact most don't,
but the most notable being when we fail to answer a call: we should
not then automatically reject it).

src/CallHandler.js

@@ -117,7 +117,6 @@ function _setCallListeners(call) {
     call.on("error", function(err) {
         console.error("Call error: %s", err);
         console.error(err.stack);
-        call.hangup();
         _setCallState(undefined, call.roomId, "ended");
         if (err.code === 'unknown_devices') {
             const QuestionDialog = sdk.getComponent("dialogs.QuestionDialog");