add useful error message if uploading file fails

I thought they are defined in db/class_file.inc.php Line 416:

When error occurs, then file upload page jumps to streber home without any message shown to user. That's the problem.

Btw. these messages should be added to translation tables.

Could you have a look at this one? Of cause giving proper (translated) errors would be nice :)

tom

I assigned this task to me just for doing this :-)