crypto/x509: improve error when PKCS1, PKCS8, EC keys are mixed up

crypto/x509: improve error when PKCS1, PKCS8, EC keys are mixed up

Improve error messages if ParsePKCS8PrivateKey/ParseECPrivateKey
/ParsePKCS1PrivateKey or ParsePKIXPublicKey/ParsePKCS1PublicKey
are called erroneously instead of one another.

Fixes #30094

Change-Id: Ia419c5f320167791aa82e174b4e9ce0f3275ec63
Reviewed-on: https://go-review.googlesource.com/c/161557
Reviewed-by: Filippo Valsorda <filippo@golang.org>
Run-TryBot: Filippo Valsorda <filippo@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>