Fix issue #6300: Improve error message for unregistered callbacks.

Summary:Fix for issue #6300:
Motivation: When more than one callback is registered to a native module, the error message that a user receives is not indicative of what is really happening.
Closes https://github.com/facebook/react-native/pull/6436

Differential Revision: D3087551

Pulled By: tadeuzagallo

fb-gh-sync-id: 93c703348dc53b75c5b507edc71754680ab5c438
shipit-source-id: 93c703348dc53b75c5b507edc71754680ab5c438

Libraries/Utilities/MessageQueue.js

@@ -192,10 +192,17 @@ class MessageQueue {
     let debug = this._debugInfo[cbID >> 1];
     let module = debug && this._remoteModuleTable[debug[0]];
     let method = debug && this._remoteMethodTable[debug[0]][debug[1]];
+    if (!callback) {
+      let errorMessage = `Callback with id ${cbID}: ${module}.${method}() not found`;
+      if (method) {
+        errorMessage = `The callback ${method}() exists in module ${module}, `
+        + `but only one callback may be registered to a function in a native module.`;
+      }
       invariant(
         callback,
-      `Callback with id ${cbID}: ${module}.${method}() not found`
+        errorMessage
       );
+    }
     let profileName = debug ? '<callback for ' + module + '.' + method + '>' : cbID;
     if (callback && SPY_MODE && __DEV__) {
       console.log('N->JS : ' + profileName + '(' + JSON.stringify(args) + ')');