Wrap blind_rsa_signature's errors in RedeemTokenError

[jcjones]

While for security purposes it's good to not pass along the details of what's failed when redeeming a token, swallowing the error here without recourse does make debugging more difficult, particularly for upstream errors like UnsupportedParameters:

privacypass/src/public_tokens/server.rs

Line 165 in b397486

.map_err(|_| RedeemTokenError::InvalidToken)?;

Perhaps the error types could use some the thiserror #[from] or #[source] mechanisms to keep the underlying error details available?

[raphaelrobert]

Makes sense, let me know if #44 works for you.