internal/ucd: fix bug

Result from Bytes was used after Scan.

Now using string internally.
Also added improved error messages.

Generated all tables and verified there are no diffs.

Change-Id: Idcb1dcb6c05c0f8dae13d5075729a5c11d611d61
Reviewed-on: https://go-review.googlesource.com/63172
Run-TryBot: Marcel van Lohuizen <mpvl@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Nigel Tao <nigeltao@golang.org>