crypto/x509: improve error when parsing bad ECDSA cert
crypto/x509: improve error when parsing bad ECDSA cert

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/57050
Reviewed-by: Filippo Valsorda <4f40cda291c5f9634e1affd3db44947af61f705c@golang.org>

cloudflare/go,Cofyc/go,josharian/go,golang/go,cloudflare/go,cloudflare/go,golang/go,josharian/go,nawawi/go,josharian/go,nawawi/go,josharian/go,nawawi/go,golang/go,Cofyc/go,akutz/go,golang/go,cloudflare/go,CAFxX/go,Cofyc/go,CAFxX/go,josharian/go,golang/go,CAFxX/go,CAFxX/go,nawawi/go,CAFxX/go,josharian/go,cloudflare/go,nawawi/go,josharian/go,nawawi/go,Cofyc/go,Cofyc/go,cloudflare/go,golang/go,akutz/go,CAFxX/go,golang/go,Cofyc/go,akutz/go,akutz/go,nawawi/go,golang/go,akutz/go,akutz/go,cloudflare/go,josharian/go,Cofyc/go,CAFxX/go,Cofyc/go,akutz/go,cloudflare/go,CAFxX/go,nawawi/go,akutz/go