Fall silently when hooks already installed.

If hooks already installed, this package will fail with error, causing whole `npm i` command to fail. To reproduce:

* run `npm i` from package using `git-hooks`, for example [bem-core](https://github.com/bem/bem-core);
* remove `node_modules` directory;
* run `npm i` again;

`npm` [best practices](https://npmjs.org/doc/scripts.html) suggests not returning non-zero code in such cases:

>Don't exit with a non-zero error code unless you really mean it. Except for uninstall scripts, this will cause the npm action to fail, and potentially be rolled back. If the failure is minor or only will prevent some optional features, then it's better to just print a warning and exit successfully.