telebot::error::Error should implement std::error::Error

[tecywiz121]

Like it says in the Title, I think telebot::error::Error should probably implement std::error::Error. That'll let it work with the error_chain crate.

[bytesnake]

Yes the library does not interop nicely with other errors. In future I want to use failure to handle the error enum, but it is not yet implemented. I'll try to update the implementation in the next days.