show crates.io search errors to the user without logging them

[syphar]

We are using the crates.io search API to power our own crate-search.

Sometimes the crates.io API returns 4xx or 5xx errors because of wrong queries or timeouts.

These errors can be shown to the user, but there is no need to report them to sentry or log them as errors.

We could add a new metric to track the amount of errors.

[syphar]

instructions:

• web::releases::get_search_results fetches the search results, it's used in web::releases::search_handler
• our logging config would send everything that is logged with tracing::error! to sentry, and also every server error
• so either the handler or the method have to be changed so we don't generate a server error any more, but just a warning.

[byfnoel]

@rustbot claim