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/common/exec.rs

@@ -13,7 +13,7 @@ pub(crate) enum Exec {
 
 
 impl Exec {
-    pub(crate) fn execute<F>(&self, fut: F)
+    pub(crate) fn execute<F>(&self, fut: F) -> ::Result<()>
     where
         F: Future<Item=(), Error=()> + Send + 'static,
     {
@@ -21,7 +21,13 @@ impl Exec {
             Exec::Default => {
                 #[cfg(feature = "runtime")]
                 {
-                    ::tokio_executor::spawn(fut)
+                    use ::tokio_executor::Executor;
+                    ::tokio_executor::DefaultExecutor::current()
+                        .spawn(Box::new(fut))
+                        .map_err(|err| {
+                            warn!("executor error: {:?}", err);
+                            ::Error::new_execute()
+                        })
                 }
                 #[cfg(not(feature = "runtime"))]
                 {
@@ -30,10 +36,11 @@ impl Exec {
                 }
             },
             Exec::Executor(ref e) => {
-                let _ = e.execute(Box::new(fut))
+                e.execute(Box::new(fut))
                     .map_err(|err| {
-                        panic!("executor error: {:?}", err.kind());
-                    });
+                        warn!("executor error: {:?}", err.kind());
+                        ::Error::new_execute()
+                    })
             },
         }
     }