Add Error interface
Hello! I am not sure if I should ask a question in the issue tracker, or is it fine to do it with an inline comment here?
The C implementation of the cipher (https://github.com/rust-bitcoin/rust-secp256k1/blob/8b4963588a4365b786be3f1fab0eb9577d7ab67c/src/lib.rs#L677) does not ever return an Error. In this pure-rust version I see that there is a 2 ^ -256 chance that a valid message and a valid signature does not produce a signature, but an error. Could you please explain, why is it like that?