Show a warning when the user has invalid library directories

Show a warning when the user has invalid library directories and remove the failback build with artifacts when library is not found

A better solution would be to replace the task called which produce "Configuration with name 'default' not found" and reword the message, but it seems a huge overkill.

A failing build will produce:
```
WARNING: /Users/max/work/automattic/WordPress-Android/libs/networking is not a valid library directory, you won't be able to build the DEBUG configuration.
WARNING: It'll cause a "Configuration with name 'default' not found" error.
WordPress debug using local source for networking :libs:networking:WordPressNetworking
WordPress release using artifact for networking org.wordpress:wordpress-networking:1.0.0
WARNING: /Users/max/work/automattic/WordPress-Android/libs/utils is not a valid library directory, you won't be able to build the DEBUG configuration.
WARNING: It'll cause a "Configuration with name 'default' not found" error.
WordPress debug using local source for utils :libs:utils:WordPressUtils
WordPress release using artifact for utils org.wordpress:wordpress-utils:1.0.1
WARNING: /Users/max/work/automattic/WordPress-Android/libs/wpcomrest is not a valid library directory, you won't be able to build the DEBUG configuration.
WARNING: It'll cause a "Configuration with name 'default' not found" error.
WordPress debug using local source for wpcomrest :libs:wpcomrest:WordPressComRest
WordPress release using artifact for wpcomrest com.automattic:wordpresscom-rest:1.0.0

FAILURE: Build failed with an exception.

* What went wrong:
A problem occurred configuring project ':WordPress'.
> Configuration with name 'default' not found.
```

Note: the release configuration will display the same warnings but should compile.