All the prefixes of the testGIF produce errors today,
but they differ wildly in which errors: some are io.EOF,
others are io.ErrUnexpectedEOF, and others are gif-specific.
Make them all gif-specific to explain context, and make
any complaining about EOF be sure to mention the EOF
is unexpected.

Fixes #11390.

Change-Id: I742c39c88591649276268327ea314e68d1de1845
Reviewed-on: https://go-review.googlesource.com/17493
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>