fix(RTC) improve bridge channel not being available error (#2533)

Claiming there is no support for it is misleading, the user might have
called the function before the channel was initialized.

modules/RTC/RTC.js

@@ -727,7 +727,7 @@ export default class RTC extends Listenable {
         if (this._channel) {
             this._channel.sendMessage(to, payload);
         } else {
-            throw new Error('Channel support is disabled!');
+            throw new Error('BridgeChannel has not been initialized yet');
         }
     }