feat for issue #809: impl std::error::Error for cursive::theme::Error

https://github.com/gyscos/cursive/issues/809 had been opened inquiring about implementing `std::error::Error` for `cursive::theme::Error` so they could do something like this: ``` fn load_theme(siv: &mut CursiveRunnable, path: PathBuf) -> anyhow::Result<()> { siv.load_theme_file(path)?; Ok(()) } ``` Instead of having to do this: ``` fn load_theme(siv: &mut CursiveRunnable, path: PathBuf) -> anyhow::Result<()> { match siv.load_theme_file(path) { Ok(_) => Ok(()), Err(e) => match e { cursive::theme::Error::Io(e) => Err(e.into()), cursive::theme::Error::Parse(e) => Err(e.into()), } } } ``` It seemed like low hanging fruit, so I figured I'd offer up a PR. Included some tests for the different `Error::Io`, `Error::Parse` and `?` cases. Thank you for considering this change!