[Repo Command] Add name of the original exception to Informative message

It could be helpful to have more details on the original exception for
troubleshoot purposes.

lib/cocoapods/command/repo/add.rb

@@ -58,7 +58,7 @@ module Pod
         def create_repos_dir
           config.repos_dir.mkpath
         rescue SystemCallError => e
-          raise Informative, "Could not create repos directory '#{config.repos_dir}'\n#{e.message}"
+          raise Informative, "Could not create repos directory '#{config.repos_dir}'.\n#{e.class.name}: #{e.message}"
         end
 
         # Clones the git spec-repo according to parameters passed to the