Change signature of `Headers::{append,insert}`

[yoshuawuyts]

See https://github.com/http-rs/tide/issues/853

Tasks

• [ ] change signature of {Request,Response,Headers}::append to return Result
• [ ] change signature of {Request,Response,Headers}::insert to return Result

[pepoviola]

Hi @yoshuawuyts, I can take a look unless you are already working on this. Thanks!

[yoshuawuyts]

@pepoviola that'd be great, all yours!