refactor(error): improve organization of Error kinds

- Placed all cases of "unexpected bytes" errors into the
  `UnexpectedMessage` variant.
- Placed all cases of "unexpected EOF" errors into the
  `IncompleteMessage` variant. Description is now generic about
  "connection closed before message completed", instead of mentioning
  "request" or "response.
- Added `Error::is_incomplete_message()` accessor to help checking for
  unexpected closures.
- Renamed some variants to be clearer when viewing the `Debug` format.
- Collected all "user" errors into an internal `User` enum, to prevent
  forgetting to update the `is_user()` method.

src/client/conn.rs

@@ -235,7 +235,7 @@ where
             },
             Err(_req) => {
                 debug!("connection was not ready");
-                let err = ::Error::new_canceled(Some("connection was not ready"));
+                let err = ::Error::new_canceled().with("connection was not ready");
                 Either::B(future::err(err))
             }
         };
@@ -262,7 +262,7 @@ where
             },
             Err(req) => {
                 debug!("connection was not ready");
-                let err = ::Error::new_canceled(Some("connection was not ready"));
+                let err = ::Error::new_canceled().with("connection was not ready");
                 Either::B(future::err((err, Some(req))))
             }
         }
@@ -322,7 +322,7 @@ where
             },
             Err(req) => {
                 debug!("connection was not ready");
-                let err = ::Error::new_canceled(Some("connection was not ready"));
+                let err = ::Error::new_canceled().with("connection was not ready");
                 Either::B(future::err((err, Some(req))))
             }
         }