Fix missing texture-download error with OpenGL ES

(cherry picked from commit 57eaea571a)

drivers/gles3/rasterizer_storage_gles3.cpp

@@ -909,7 +909,7 @@ Ref<Image> RasterizerStorageGLES3::texture_get_data(RID p_texture, VS::CubeMapSi
 #else
 
 	ERR_EXPLAIN("Sorry, It's not possible to obtain images back in OpenGL ES");
-	return Ref<Image>();
+	ERR_FAIL_V(Ref<Image>());
 #endif
 }