Don't capture client errors in Actix Web integration

[lcian]

Due to this code path, when polling the future returns an error, we capture it only based on capture_server_errors.

This can happen, for example, when a middleware processes the request after ours, and returns a actix_web::error::Error.
The problem is that the middleware could be, for instance, an authentication middleware, that returns 401 (a client error).
We would capture this error because we don't check if it's a client or server error.

[linear]

RUST-95 Don't capture client errors in Actix Web integration