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/pool.rs

@@ -577,14 +577,14 @@ impl<T: Poolable> Checkout<T> {
                     if value.is_open() {
                         Ok(Async::Ready(Some(self.pool.reuse(&self.key, value))))
                     } else {
-                        Err(::Error::new_canceled(Some(CANCELED)))
+                        Err(::Error::new_canceled().with(CANCELED))
                     }
                 },
                 Ok(Async::NotReady) => {
                     self.waiter = Some(rx);
                     Ok(Async::NotReady)
                 },
-                Err(_canceled) => Err(::Error::new_canceled(Some(CANCELED))),
+                Err(_canceled) => Err(::Error::new_canceled().with(CANCELED)),
             }
         } else {
             Ok(Async::Ready(None))
@@ -654,7 +654,7 @@ impl<T: Poolable> Future for Checkout<T> {
         if let Some(pooled) = self.checkout() {
             Ok(Async::Ready(pooled))
         } else if !self.pool.is_enabled() {
-            Err(::Error::new_canceled(Some("pool is disabled")))
+            Err(::Error::new_canceled().with("pool is disabled"))
         } else {
             Ok(Async::NotReady)
         }