Improve error handling when reading PKCS8 private key

When reading a pkcs8 key this exception is thrown:

  File "/usr/lib/python2.7/site-packages/pyXMLSecurity-0.16.dev0-py2.7.egg/xmlsec/rsa_x509_pem/__init__.py", line 39, in parse
    raise Exception("PEM data type not supported.")

Expected message: "PKCS8 private key format not supported".

This happens when creating RSA key pairs with `openssl req` 