gears.object: Give better error messages

It helps a lot to know which signal does not exist. That should make it a lot
easier to look for the "guily" code without having to resort to the backtrace.

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

lib/gears/object.lua.in

@@ -27,7 +27,7 @@ end
 local function find_signal(obj, name, error_msg)
     check(obj)
     if not obj._signals[name] then
-        error("Trying to " .. error_msg .. " non-existent signal")
+        error("Trying to " .. error_msg .. " non-existent signal '" .. name .. "'")
     end
     return obj._signals[name]
 end