rcodesign should error if no private key pem file is specified

[amgdev9]

If I do rcodesign sign --pem-file cert.pem MyApp.app, being cert.pem a regular certificate pem file, the tool should print an error indicating that no private key has been specified. Right now it just goes on and leaves the app executable partially unsigned.

[indygreg]

I think that providing a public certificate without its signing key should be a fatal error. Thanks for filing the issue!