Changed function to `not_found`.

Instead of using funtion `error!` the function `not_found!` is used to return 404 error. Adjusted documentation accordingly.

doc/api/repositories.md

@@ -79,6 +79,9 @@ Parameters:
 }
 ```
 
+Will return status code `200` on success or `404 Not found` if the branch is not available.
+
+
 ## Protect a project repository branch
 
 Protect a single project repository branch.

lib/api/projects.rb

@@ -230,7 +230,7 @@ module Gitlab
       #   GET /projects/:id/repository/branches/:branch
       get ":id/repository/branches/:branch" do
         @branch = user_project.repo.heads.find { |item| item.name == params[:branch] }
-        error!("Branch does not exist", 404) if @branch.nil?
+        not_found!("Branch does not exist") if @branch.nil?
         present @branch, with: Entities::RepoObject, project: user_project
       end