unclear error when tests fail due to lack of openssl

[jelly]

I'm packaging in-toto-rs now to get rebuilderd in Fedora and it took me a while to figure out openssl is needed for the tests, maybe the error can be expanded to mention missing openssl?

---- crypto::test::new_rsa_key stdout ----

thread 'crypto::test::new_rsa_key' panicked at src/crypto.rs:1803:51:
called `Result::unwrap()` on an `Err` value: NotFound
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

[adityasaky]

cc @alanssitis