Reduce duplication in GitAccess spec around error messages

- Adds a new `ProjectMovedError` class to encapsulate that error
  condition. Inherits from `NotFoundError` so existing rescues should
  continue to work.
- Separating that condition out of `NotFoundError` allowed us to
  simplify the `raise_not_found` helper and avoid repeating the literal
  string.
- Spec makes use of `ERROR_MESSAGES` hash to avoid repeating literal
  error message strings.

lib/gitlab/git_access.rb

@@ -4,6 +4,7 @@ module Gitlab
   class GitAccess
     UnauthorizedError = Class.new(StandardError)
     NotFoundError = Class.new(StandardError)
+    ProjectMovedError = Class.new(NotFoundError)
 
     ERROR_MESSAGES = {
       upload: 'You are not allowed to upload code for this project.',
@@ -90,7 +91,8 @@ module Gitlab
     end
 
     def check_project_moved!
-      if redirected_path
+      return unless redirected_path
+
       url = protocol == 'ssh' ? project.ssh_url_to_repo : project.http_url_to_repo
       message = <<-MESSAGE.strip_heredoc
         Project '#{redirected_path}' was moved to '#{project.full_path}'.
@@ -100,8 +102,7 @@ module Gitlab
           git remote set-url origin #{url}
       MESSAGE
 
-        raise NotFoundError, message
-      end
+      raise ProjectMovedError, message
     end
 
     def check_command_disabled!(cmd)