mesa: Add _mesa_lookup_vao_err

This is a convenience function that generates GL_INVALID_OPERATION
when the array object doesn't exist.

Reviewed-by: Laura Ekstrand <laura@jlekstrand.net>

src/mesa/main/arrayobj.h

@@ -48,6 +48,9 @@ struct gl_context;
 extern struct gl_vertex_array_object *
 _mesa_lookup_vao(struct gl_context *ctx, GLuint id);
 
+extern struct gl_vertex_array_object *
+_mesa_lookup_vao_err(struct gl_context *ctx, GLuint id, const char *caller);
+
 extern struct gl_vertex_array_object *
 _mesa_new_vao(struct gl_context *ctx, GLuint name);