use raising validate function in viewport

Author: vacarsu

lib/scenic/view_port.ex

@@ -395,7 +395,7 @@ defmodule Scenic.ViewPort do
   def set_theme(viewport, theme)
 
   def set_theme(%ViewPort{pid: pid}, theme) do
-    case Themes.validate(theme) do
+    case Themes.validate!(theme) do
       # {:ok, theme} -> GenServer.cast( pid, {:set_theme, theme} )
       {:ok, theme} -> GenServer.call(pid, {:set_theme, theme})
       err -> err