fix(lib): return an error instead of panic if execute fails

If executing an internal task fails, a new variant of `hyper::Error` is
returned to the user, with improved messaging.

If a non-critical task fails to spawn, it no longer panics, instead just
logging a warning.

Closes #1566

src/server/conn.rs

@@ -641,7 +641,7 @@ where
                     // flatten basically
                     .and_then(|conn| conn.with_upgrades())
                     .map_err(|err| debug!("conn error: {}", err));
-                self.serve.protocol.exec.execute(fut);
+                self.serve.protocol.exec.execute(fut)?;
             } else {
                 return Ok(Async::Ready(()))
             }