Check that hooks directory exists before attempting to call realpath

This MR checks that the hooks directories actually exist before attempting to resolve their `realpath`.

Users who attempted to restore from source to an omnibus installation would get ugly errors when running `gitlab-rake gitlab:check`:

```
Errno::ENOENT: No such file or directory @ realpath_rec - /var/opt/gitlab/git-data/repositories/Wanda/www.git/hooks
/opt/gitlab/embedded/service/gitlab-rails/lib/tasks/gitlab/check.rake:488:in `realpath'
/opt/gitlab/embedded/service/gitlab-rails/lib/tasks/gitlab/check.rake:488:in `block in check_repos_hooks_directory_is_link'
/opt/gitlab/embedded/service/gem/ruby/2.1.0/gems/activerecord-4.1.11/lib/active_record/relation/batches.rb:52:in `block (2 levels) in find_each'
/opt/gitlab/embedded/service/gem/ruby/2.1.0/gems/activerecord-4.1.11/lib/active_record/relation/batches.rb:52:in `each'
/opt/gitlab/embedded/service/gem/ruby/2.1.0/gems/activerecord-4.1.11/lib/active_record/relation/batches.rb:52:in `block in find_each'
/opt/gitlab/embedded/service/gem/ruby/2.1.0/gems/activerecord-4.1.11/lib/active_record/relation/batches.rb:126:in `find_in_batches'
/opt/gitlab/embedded/service/gem/ruby/2.1.0/gems/activerecord-4.1.11/lib/active_record/relation/batches.rb:51:in `find_each'
/opt/gitlab/embedded/service/gem/ruby/2.1.0/gems/activerecord-4.1.11/lib/active_record/querying.rb:9:in `find_each'
/opt/gitlab/embedded/service/gitlab-rails/lib/tasks/gitlab/check.rake:482:in `check_repos_hooks_directory_is_link'
/opt/gitlab/embedded/service/gitlab-rails/lib/tasks/gitlab/check.rake:343:in `block (3 levels) in <top (required)>'
Tasks: TOP => gitlab:check => gitlab:gitlab_shell:check
```

Closes #2121

#2082

See merge request !1068