Improve error handling for nonexistent channels

Dependencies

• [2] GYXIF25T Proper parsing of URLs
• [3] CCLLB7OI Upgrading to Sanakirja 0.15 + version bump
• [4] TPEH2XNB 1.0.0-alpha.28, with Tokio 1.0
• [5] JRENVH5D Reqwest 0.11
• [6] MU5GSJAW Partial push and pull (WARNING: breaks the existing protocol)
• [*] FBXYP7QM Forgot to add remote::http

Change contents

• replacement in pijul/src/remote/http.rs at line 182
  [3.207]→[3.5653:5693](∅→∅),[3.1457]→[3.5653:5693](∅→∅),[2.1924]→[3.5653:5693](∅→∅),[3.5653]→[3.5653:5693](∅→∅),[3.5693]→[3.11188:11239](∅→∅)
  − if !res.status().is_success() {

  − bail!("HTTP error {:?}", res.status())
  [2.1924]

  [3.5813]
  + let status = res.status();

  + if !status.is_success() {

  + match serde_json::from_slice::<libpijul::RemoteError>(&*res.bytes().await?) {

  + Ok(remote_err) => return Err(remote_err.into()),

  + Err(_) if status.as_u16() == 404 => bail!("Repository `{}` not found (404)", self.url),

  + Err(_) => bail!("Http request failed with status code: {}", status)

  + }