addOnNavigatorEvent improvements (#2175)

* throwing an error instead of a string

* return a callback to remove the register handler

docs/screen-api.md

 ## setOnNavigatorEvent(callback)
 
 Set a handler for navigator events (like nav button press). This would normally go in your component constructor.
+Can not be used in conjuction with `addOnNavigatorEvent`.
 
 ```js
 // this.onNavigatorEvent will be our handler
 this.props.navigator.setOnNavigatorEvent(this.onNavigatorEvent.bind(this));
 ```
 
+## addOnNavigatorEvent(callback)
+
+Add a handler for navigator events (like nav button press). This would normally go in your component constructor.
+If you choose to use `addOnNavigatorEvent` instead of `setOnNavigatorEvent` you will be able to add multiple handlers.
+Bear in mind that you can't use both `addOnNavigatorEvent` and `setOnNavigatorEvent`.
+`addOnNavigatorEvent` returns a function, that once called will remove the registered handler.
+
 # Screen Visibility
 
 `const isVisible = await this.props.navigator.screenIsCurrentlyVisible()`

src/Screen.js

 
   setOnNavigatorEvent(callback) {
     if (this.navigatorEventHandlers.length > 0) {
-      throw 'setOnNavigatorEvent can not be used after addOnNavigatorEvent has been called';
+      throw new Error('setOnNavigatorEvent can not be used after addOnNavigatorEvent has been called');
     }
     this.navigatorEventHandler = callback;
     this._registerNavigatorEvent();
 
   addOnNavigatorEvent(callback) {
     if (this.navigatorEventHandler) {
-      throw 'addOnNavigatorEvent can not be used after setOnNavigatorEvent has been called';
+      throw new Error('addOnNavigatorEvent can not be used after setOnNavigatorEvent has been called');
     }
     if (this.navigatorEventHandlers.indexOf(callback) === -1) {
       this.navigatorEventHandlers.push(callback);
     }
     this._registerNavigatorEvent();
+
+    return () => this._removeOnNavigatorEvent(callback)
+    
   }
 
   _registerNavigatorEvent() {
     }
   }
 
-  removeOnNavigatorEvent(callback) {
+  _removeOnNavigatorEvent(callback) {
     const index = this.navigatorEventHandlers.indexOf(callback);
     if (index !== -1) {
       this.navigatorEventHandlers.splice(index, 1);