Add an error message when using shell commands with incompatible GitLab versions

CHANGELOG

 v3.2.0
   - Allow GitLab Shell to check for allowed access based on the used Git protocol
+  - Add an error message when using shell commands with incompatible GitLab versions
 
 v3.1.0
   - Refactor repository paths handling to allow multiple git mount points

lib/gitlab_shell.rb

@@ -182,6 +182,7 @@ class GitlabShell
   private
 
   def repo_path=(repo_path)
+    raise ArgumentError, "Repository path not provided. Please make sure you're using GitLab v8.10 or later." unless repo_path
     raise InvalidRepositoryPathError if File.absolute_path(repo_path) != repo_path
 
     @repo_path = repo_path