Improve the error message for the current breakage (#2749)

The tests for naughty are currently broken, because naughty.dbus now
uses Gio for interacting with DBus, but the tests still try to use
awesome's dbus object. Turn the resulting weird error into something a
lot less cryptic.

Signed-off-by: Uli Schlachter <psychon@znc.in>

tests/examples/naughty/template.lua

@@ -18,6 +18,14 @@ end
 
 local rect = {x1 = math.huge ,y1 = math.huge , x2 = -math.huge , y2 = -math.huge}
 
+if #drawin.get() == 0 then
+    -- The code below would calculate -math.huge - math.huge and end up trying
+    -- to draw something of size -math.huge times -math.huge. Turn this weird
+    -- error into a better error message.
+    error("Got no drawins to draw. "
+            .. "That's not good and would cause errors later on.")
+end
+
 -- Get the region with wiboxes
 for _, d in ipairs(drawin.get()) do
     local w = d.get_wibox and d:get_wibox() or nil