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/proto/h2/server.rs

@@ -132,7 +132,7 @@ where
                 ::Body::h2(stream, content_length)
             });
             let fut = H2Stream::new(service.call(req), respond);
-            exec.execute(fut);
+            exec.execute(fut)?;
         }
 
         // no more incoming streams...