keymgr: Decide whether to allow `KeyStore` implementers to bring their own error type

We have an Error enum in tor-keymgr that keeps growing keystore-specific variants (for example, the variants related to OpenSSH parsing issues are only ever returned by ArtiKeyNativeStore). If we don't allow external KeyStore implementers to bring their own error type, they're stuck picking from the error variants we defined (which will all probably be unsuitable as they're tailored to our own KeyStore implementations).

So should each KeyStore impl be able to bring its own error type? That would mean returning Result<..., Box<dyn Error>> from the KeyStore functions (instead of Result<..., tor_keymgr::Error>). However, using an opaque error type will make handling KeyMgr errors difficult: when the KeyMgr bubbles up a KeyStore error, the caller has to downcast it to a specific type (this assumes the caller knows which keystore the error came from, and thus what error type to downcast to. To make this work, we could make the error type be Box<dyn OurErrorTrait> and have OurErrorTrait specify what keystore the error came from, for example via a OurErrorTrait::keystore_name() method).