more accurate error message

[philippecery]

Hi @youmark,

In issue #17, @Fire1nRain seemed confused by the error message "only PKCS#5 v2.0 supported". Indeed, the error occured, not because PKCS#5 v2.0 was not supported - based on the meta data, that private key seemed encrypted using PKCS#5 v2.0 - but because he passed an invalid ASN1 block - most likely, as @conradoplg mentioned, the raw PEM he read from the file - which is why asn1.Unmarshal failed.
So, I suggest this change to the error message (1) to be consistent with the error message returned by x509.ParsePKCS8PrivateKey when asn1.Unmarshal fails and (2) to help developers who inadvertently passed an invalid ASN1 and think it is a bug.

Hope that helps clarify things. Thanks for this library, very helpful.

Regards,
Philippe.

[UlrichEckhardt]

I've run the tests on this and it doesn't cause regressions. I like the fact that it also adds a bunch of test cases. I'd vote for merging this PR.