gears.timer: Don't assert() that no error occured

The whole point of this pcall() is that we do not have unprotected Lua errors,
because those kill awesome. So instead of assert()ing, let's just print a
message.

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

lib/gears/timer.lua.in

@@ -33,7 +33,9 @@ function timer:start()
         local success, message = pcall(function()
             self:emit_signal("timeout")
         end)
-        assert(success, message)
+        if not success then
+            print(message)
+        end
         return true
     end)
 end