`HttpPeer::new`. Return `Result<Self>` instead of `panic`

[cospectrum]

Relates: https://github.com/cloudflare/pingora/issues/570

[cospectrum]

sfv = "0" dependency breaks the build

[matthewbjones]

@cospectrum I'm curious to hear why you closed this PR. This is something we've also experienced in our production instance of Pingora (throwing a panic when an HttpPeer's DNS temporarily isn't resolving) Ideally this would be more Result-driven and our implementation of the proxy can choose what to do in this scenario (return a proper 502 error page, etc.)

[drcaramelsyrup]

@cospectrum This looks reasonable. I don't think the documentation whitespace changes were intentional though?

[cospectrum]

@drcaramelsyrup check this out ^