Raise exception of blend_func issue

Signed-off-by: Emmanuel Ferdman <emmanuelferdman@gmail.com>

Author: emmanuel-ferdman

arcade/gl/backends/opengl/context.py

@@ -143,7 +143,7 @@ def blend_func(self, value: Tuple[int, int] | Tuple[int, int, int, int]):
         elif len(value) == 4:
             gl.glBlendFuncSeparate(*value)
         else:
-            ValueError("blend_func takes a tuple of 2 or 4 values")
+            raise ValueError("blend_func takes a tuple of 2 or 4 values")
 
     @property
     def front_face(self) -> str: