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/client/pool.rs

@@ -407,12 +407,16 @@ impl<T: Poolable> Connections<T> {
 
         let start = Instant::now() + dur;
 
-        let interval = Interval::new(start, dur);
-        self.exec.execute(IdleInterval {
-            interval: interval,
+        let interval = IdleInterval {
+            interval: Interval::new(start, dur),
             pool: WeakOpt::downgrade(pool_ref),
             pool_drop_notifier: rx,
-        });
+        };
+
+        if let Err(err) = self.exec.execute(interval) {
+            // This task isn't critical, so simply log and ignore.
+            warn!("error spawning connection pool idle interval: {}", err);
+        }
     }
 }