Improve error message for missing plugin with regenerate

When adding a new entry to an existing manifest and then calling `regenerate` without first clone the new repository, the tool just complains about a failing `lstat`:

```
Unable to eval symlinks for /home/user/graylog/graylog-project-repos/graylog-plugin-foo: lstat /home/user/graylog/graylog-project-repos/graylog-plugin-foo: no such file or directory
```

Make this a bit friendlier to explain what happened (and suggest that a `checkout` might solve this problem).