crypto/x509: offer a useful error when ParsePKCS8PrivateKey/ParseECPrivateKey/ParsePKCS1PrivateKey or ParsePKIXPublicKey/ParsePKCS1PrivateKey are mixed up

[FiloSottile]

When parsing fails in one of these functions, we could check if one of the others would have worked, and offer a helpful error message.

[arashbina]

I can work on this.
So if I understand correctly when unmarshalling to one structure if it fails try the counterpart to see if it succeeds and return a descriptive error.

[FiloSottile]

Correct. Please add me to the reviewers for the change. Thank you!

[gopherbot]

Change https://golang.org/cl/161557 mentions this issue: x509: improve error when PKCS1, PKCS8, EC, PKIX keys are mixed up