Improve and simplify commands errors

[Urgau]

I had to fought today to find the error in rust-lang/rustup#2876 (comment), mainly because CloudWatch is really bad, but also because we didn't post any error to the user.

This PR improves the way we deal with user facing error by creating a inform! macro that wraps anyhow::bail!+UserError. This allows two things, first that we always post an error message to the user and second that we log it (correctly).

This has also the side-effect that don't return 500 when we only have user facing errors. It also reduce the boilerplate of doing ErrorComment+post()+return Ok(()) each time.

To put it in another way, let's use our the HandlerError infra the way it should be used.

[Kobzol]

I'm not a big fan of control-flow being hidden inside macros, so I'd rather do return user_error!(...);. But I'm also fine with the current version.

View changes since this review

[Urgau]

Agree with you that the hidden control-flow was a bit weird. Changed to your proposed user_error! macro.