tests: Test the error notifications when no screens are present.

This happens in early initialization when AwesomeWM is started with
`--screen manual`.

tests/test-awful-screen.lua

@@ -64,6 +64,23 @@ local steps = {
 
             return true
         end
+    end,
+    -- Make sure the error code still works when all screens are gone.
+    function()
+        while screen.count() > 0 do
+            screen[1]:fake_remove()
+        end
+
+        -- Don't make the test fail.
+        local called = false
+        require("gears.debug").print_warning = function() called = true end
+
+        -- Cause an error in a protected call!
+        awesome.emit_signal("debug::error", "err")
+
+        assert(called)
+
+        return true
     end
 }