making build fail when duplicate package detected in default case

* added property `failOnDuplicatePackages` 
* made build fail in default case
* corrected output message <br>
based on <a href="https://github.com/william-ferguson-au/maven-android-plugin/commit/833a9c1b078fa8bd97eae78257e0c8b63f723857#commitcomment-6348645">recommendation from William Ferguson</a>
inspired by #352