ocsp: enhance errors for responses without a matching SingleResponse

ParseResponseForCert would previously complain about an invalid hash OID
if the response contained no SingleResponse matching the certificate
provided by the caller.

Fixes golang/go#19540

Change-Id: I0354c4048707a788ed3d184cc88b4f13f65544ba
Reviewed-on: https://go-review.googlesource.com/44005
Reviewed-by: Adam Langley <agl@golang.org>
2 files changed