Until now we've used ErrUnknownAlgorithm but that's a bit confusing
when it is returned for obviously-known things like MD5.

Fixes #10431.

Change-Id: Ief8a8ef46e5b99bd4fd18e1acd7ae398a484bac3
Reviewed-on: https://go-review.googlesource.com/17380Reviewed-by: Adam Langley <agl@golang.org>