Moodle's errors are usually pretty good as long as you have Developer level debugging enabled. Sometimes you have to have a look at the code to see what's going on.
If you do get a truly useless message (I would sack anybody who coded "an unknown error occurred" ) please log it in the bug tracker.
If you do get a truly useless message (I would sack anybody who coded "an unknown error occurred" ) please log it in the bug tracker.