API: groups documentation updated, functions return different status codes

Updates the API documentation of groups with infos to return codes. The function calls
in the groups API have updated documentation and return `400 Bad Request` status code
if a required attribute is missing.

lib/api/groups.rb

@@ -6,6 +6,9 @@ module Gitlab
     resource :groups do
       # Get a groups list
       #
+      # Parameters
+      #   page (optional) - The page number of the groups list
+      #   per_page (optional) - The number of elements per page
       # Example Request:
       #  GET /groups
       get do
@@ -20,12 +23,16 @@ module Gitlab
       # Create group. Available only for admin
       #
       # Parameters:
-      #   name (required)                   - Name
-      #   path (required)                   - Path
+      #   name (required) - The name of the group
+      #   path (required) - The path of the group
       # Example Request:
       #   POST /groups
       post do
         authenticated_as_admin!
+
+        bad_request!(:name) unless params[:name].present?
+        bad_request!(:path) unless params[:path].present?
+
         attrs = attributes_for_keys [:name, :path]
         @group = Group.new(attrs)
         @group.owner = current_user