Remove circular dependency between HTTPHelper and GitlabNet

HttpHelper depended on exceptions defined in GitLabNet, but GitLabNet
included HttpHelper. Specs in Ruby 2.5 were breaking as a result of
Action::Custom not able to reference constants in GitLabNet:
https://gitlab.com/gitlab-org/gitlab-shell/-/jobs/126362702.

Closes https://gitlab.com/gitlab-org/gitlab-shell/issues/169

lib/gitlab_net.rb

@@ -10,9 +10,6 @@ require_relative 'http_helper'
 class GitlabNet # rubocop:disable Metrics/ClassLength
   include HTTPHelper
 
-  class ApiUnreachableError < StandardError; end
-  class NotFound < StandardError; end
-
   CHECK_TIMEOUT = 5
   API_INACCESSIBLE_MESSAGE = 'API is not accessible'.freeze
 

lib/gitlab_net/errors.rb

+class GitlabNet
+  class ApiUnreachableError < StandardError; end
+  class NotFound < StandardError; end
+end

lib/http_helper.rb

 require_relative 'httpunix'
 require_relative 'gitlab_logger'
+require_relative 'gitlab_net/errors'
 
 module HTTPHelper
   READ_TIMEOUT = 300