Improve error messages related to failing to open files

(cherry picked from commit bf8f763524d6959f0bdaca31be077fbac0d73678)

scene/resources/texture.cpp

@@ -463,7 +463,7 @@ Error StreamTexture::_load_data(const String &p_path, int &tw, int &th, int &tw_
 	ERR_FAIL_COND_V(image.is_null(), ERR_INVALID_PARAMETER);
 
 	FileAccess *f = FileAccess::open(p_path, FileAccess::READ);
-	ERR_FAIL_COND_V(!f, ERR_CANT_OPEN);
+	ERR_FAIL_COND_V_MSG(!f, ERR_CANT_OPEN, vformat("Unable to open file: %s.", p_path));
 
 	uint8_t header[4];
 	f->get_buffer(header, 4);