Implement Error for SSDPError

[GGist]

This requires removing the generic impl<T> From<T> for SSDPError where T: Error + 'static so we would have to implement From for each error type manually. Still deciding whether we should expose external crate errors by their full type or polymorphically via Box<Error>.

This may require introducing an SSDPError struct that stores the description as a string and converting the current SSDPError to an SSDPErrorKind with all of it's internal data intact (possibly private if that is even possible?).