Correct handling of nonexistent files in maven-plugin

Nonexistent files included in "import" declarations of maven-plugin are silently ignored. I think it's erroneous behavior that may be fixed easily. It already caused bugs in our company and I see that we're not the only ones.

I've created a PR from branch with testcase and working fix. I will be grateful if you can take a look on that and either merge or suggest other solution.