ref(JitsiConference): mention ping type and include the error message

There are different types of pings in the app, so it's good to mention
which one is the error being logged for. Also extracts the error
message to give any details about the error.

JitsiConference.js

@@ -329,7 +329,7 @@ JitsiConference.prototype._init = function(options = {}) {
                 this.sendMessage(
                     message, to, true /* sendThroughVideobridge */);
             } catch (error) {
-                logger.warn('Failed to send a ping request or response.');
+                logger.warn('Failed to send E2E ping request or response.', error && error.msg);
             }
         });