Implement Error trait for error types

[jmacdonald]

Implementing Rust's canonical Error trait would allow errors in syntect to be chained with application-level errors, via error-chain.

[Enselic]

For the record and FWIW: the error-chain crate has been deprecated. The new de-facto standard for libraries seems to be the thiserror crate. We recently replaced error-chain with thiserror in bat. See sharkdp/bat#1820

[Enselic]

I'd like to take a shot at this for 5.0.0.