Exit with non-zero status on disabled gem system update.

Merged: https://github.com/ruby/ruby/pull/3229

lib/rubygems/commands/update_command.rb

@@ -264,7 +264,7 @@ command to remove old versions.
   def update_rubygems
     if Gem.disable_system_update_message
       alert_error Gem.disable_system_update_message
-      return
+      terminate_interaction 1
     end
 
     check_update_arguments

test/rubygems/test_gem_commands_update_command.rb

@@ -245,8 +245,10 @@ class TestGemCommandsUpdateCommand < Gem::TestCase
     @cmd.options[:args] = []
     @cmd.options[:system] = true
 
-    use_ui @ui do
-      @cmd.execute
+    assert_raises Gem::MockGemUi::TermError do
+      use_ui @ui do
+        @cmd.execute
+      end
     end
 
     assert_empty @ui.output