Better expect error messages (#2629)

* Replace expect error messages

* Change message

* Create RuntimeError struct

* Pass error through core methods

* Fix use of Runtime::current in signals package

* Fix tests

* Add #[track_caller] for better error output and fix maybe_with_rt

* provide a help message along with RuntimeError

---------

Co-authored-by: Evan Almloff <evanalmloff@gmail.com>

packages/signals/src/global/signal.rs

@@ -68,7 +68,7 @@ impl<T: 'static> GlobalSignal<T> {
 
     #[doc(hidden)]
     pub fn maybe_with_rt<O>(&self, f: impl FnOnce(&T) -> O) -> O {
-        if Runtime::current().is_none() {
+        if Runtime::current().is_err() {
             f(&(self.initializer)())
         } else {
             self.with(f)