refactor(error): improve error message when tokio::spawn fails

Closes #1635

src/error.rs

@@ -258,8 +258,8 @@ impl Error {
         Error::new(Kind::Shutdown, Some(Box::new(cause)))
     }
 
-    pub(crate) fn new_execute() -> Error {
-        Error::new(Kind::Execute, None)
+    pub(crate) fn new_execute<E: Into<Cause>>(cause: E) -> Error {
+        Error::new(Kind::Execute, Some(cause.into()))
     }
 
     pub(crate) fn new_h2(cause: ::h2::Error) -> Error {
@@ -269,10 +269,12 @@ impl Error {
 
 impl fmt::Debug for Error {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
-        f.debug_struct("Error")
-            .field("kind", &self.inner.kind)
-            .field("cause", &self.inner.cause)
-            .finish()
+        let mut f = f.debug_struct("Error");
+        f.field("kind", &self.inner.kind);
+        if let Some(ref cause) = self.inner.cause {
+            f.field("cause", cause);
+        }
+        f.finish()
     }
 }