image/tiff: replace repeated error creations with one

Make all occurrences of "not enough pixel data" refer to the
same one.

Change-Id: Iecff9f22eba7a8852174ba18d1e68226c76f84c6
Reviewed-on: https://go-review.googlesource.com/11030
Reviewed-by: Nigel Tao <nigeltao@golang.org>
1 file changed