improve exception message when we can't find a file

Summary:
in ```fastfs.js ``` when ```getFile()``` got a exception it will print ``` Unable to find file with path: null ``` in terminal .

It's  confused
Closes https://github.com/facebook/react-native/pull/4737

Reviewed By: svcscm

Differential Revision: D2752888

Pulled By: androidtrunkagent

fb-gh-sync-id: a366da1eea27c691248dcb17019f4462a639ea70

packager/react-packager/src/DependencyResolver/fastfs.js

@@ -109,7 +109,7 @@ class Fastfs extends EventEmitter {
   readFile(filePath) {
     const file = this._getFile(filePath);
     if (!file) {
-      throw new Error(`Unable to find file with path: ${file}`);
+      throw new Error(`Unable to find file with path: ${filePath}`);
     }
     return file.read();
   }