fix(core): errored initial snapshot throws sync w/o observers

This modifies the error-handling logic for when an error is thrown during the creation of an Actor's initial snapshot (during `start()`).  _If_ the actor has _no_ observers, the error will now be thrown synchronously out of `start()` instead of to the global error handler.

Example use case:

```js

const actor = createBrokenActor();
const machine = createMachine({
  context: () => {
    throw new Error('egad!');
  }
});

try {
  await toPromise(actor.start());
} catch (err) {
  err.message === 'egad!' // true
}
```

Fixes: statelyai#4928

packages/core/src/createActor.ts

@@ -491,7 +491,26 @@ export class Actor<TLogic extends AnyActorLogic>
         // TODO: rethink cleanup of observers, mailbox, etc
         return this;
       case 'error':
+        // in this case, creation of the initial snapshot caused an error.
+
+        // **if the actor has no observer when start() is called**, this error would otherwise be
+        // thrown to the global error handler. to prevent this--and allow start() to be wrapped in try/catch--we
+        // create a temporary observer that receives the error (via _reportError()) and rethrows it from this call stack.
+        let err: unknown;
+        let errorTrapSub: Subscription | undefined;
+        if (!this.observers.size) {
+          errorTrapSub = this.subscribe({
+            error: (error) => {
+              // we cannot throw here because it would be caught elsewhere and rethrown as unhandled
+              err = error;
+            }
+          });
+        }
         this._error((this._snapshot as any).error);
+        errorTrapSub?.unsubscribe();
+        if (err) {
+          throw err;
+        }
         return this;
     }
 

packages/core/test/errors.test.ts

@@ -5,7 +5,8 @@ import {
   createMachine,
   fromCallback,
   fromPromise,
-  fromTransition
+  fromTransition,
+  toPromise
 } from '../src';
 
 const cleanups: (() => void)[] = [];
@@ -892,4 +893,41 @@ describe('error handling', () => {
       error_thrown_in_guard_when_transitioning]
     `);
   });
+
+  it('error thrown when resolving the initial context should rethrow synchronously', (done) => {
+    const machine = createMachine({
+      context: () => {
+        throw new Error('oh no');
+      }
+    });
+
+    const actor = createActor(machine);
+
+    installGlobalOnErrorHandler(() => {
+      done.fail();
+    });
+
+    expect(() => actor.start()).toThrowErrorMatchingInlineSnapshot(`"oh no"`);
+
+    setTimeout(() => {
+      done();
+    }, 10);
+  });
+
+  it('error thrown when resolving the initial context should reject when wrapped in a Promise', async () => {
+    const machine = createMachine({
+      context: () => {
+        throw new Error('oh no');
+      }
+    });
+
+    const actor = createActor(machine);
+
+    try {
+      await toPromise(actor.start());
+      fail();
+    } catch (err) {
+      expect((err as Error).message).toEqual('oh no');
+    }
+  });
 });