Reuse proxypair if sendAnswer fails

Make sure to set proxypair.active to false if createAnswer or
setLocalDescription fails. This should prevent one edge case the results
in an infinite loop described in ticket #31100.

proxy/snowflake.js

@@ -120,10 +120,11 @@ class Snowflake {
     var fail, next;
     next = function(sdp) {
       dbg('webrtc: Answer ready.');
-      return pair.pc.setLocalDescription(sdp);
+      return pair.pc.setLocalDescription(sdp).catch(fail);
     };
     fail = function() {
-      return dbg('webrtc: Failed to create Answer');
+      pair.active = false
+      return dbg('webrtc: Failed to create or set Answer');
     };
     return pair.pc.createAnswer().then(next).catch(fail);
   }