When parsing an ECDSA certificate, improve the error message upon
failing to parse the curve as a named curve, rather than returning
the original ASN1 error.

Fixes #21502

Change-Id: I7ae7b3ea7a9dcbd78a9607f46f5883d3193b8367
Reviewed-on: https://go-review.googlesource.com/57050Reviewed-by: Filippo Valsorda <filippo@golang.org>