lib/tasks/gitlab/check: Exit with non-zero code, if something failed in a check task

This is handy for monitoring tools, which could e.g. periodically call check
tasks and instead of parsing output, rely on exit code.

The way we detect if something failed is via hooking into String#red, and if
anything was ever printed in red - that's an error.

lib/tasks/gitlab/check.rake

+# if we ever print anything in red - that's an error
+$check_failed = false
+class String
+  alias_method :orig_red, :red
+  def red(*args)
+    $check_failed = true
+    orig_red(*args)
+  end
+end
+
 namespace :gitlab do
   desc "GitLab | Check the configuration of GitLab and its environment"
   task check: %w{gitlab:gitlab_shell:check
@@ -848,8 +858,9 @@ namespace :gitlab do
 
   def finished_checking(component)
     puts ""
-    puts "Checking #{component.yellow} ... #{"Finished".green}"
+    puts "Checking #{component.yellow} ... #{$check_failed ? "Failed".red : "OK".green}"
     puts ""
+    exit 1 if $check_failed
   end
 
   def see_database_guide