crypto bindings very ugly panic

`bindings::crypto::sign::sign_detached`, called from lua with

~~~lua
crypto.sign.load_secret(priv_key_string):sign_detached(value_to_sign)
~~~

panics very ugly when the value to be signed is not a string. I imagine other crypto functions behave the same way. They should throw a Lua error detailing what happened instead of rust-panicking the thread.