Implement core::error::Error for all error types

[PhilippGackstatter]

Implements core::error::Error for all error types in a non-breaking way, which is why this targets main.

The motivation is to be able to wrap these errors in Box<dyn Error> and use them as source errors which requires the trait to be implemented.

Also fixes a clippy lint introduced in a recent rust nightly version replacing string.as_bytes().len() with string.len().

[irakliyk]

Looks good! Thank you!