fix: MuxAsyncIterator throws muxed errors (#6295)

Fixes #5260

std/async/mux_async_iterator.ts

@@ -7,14 +7,15 @@ interface TaggedYieldedValue<T> {
 }
 
 /** The MuxAsyncIterator class multiplexes multiple async iterators into a
- * single stream. It currently makes a few assumptions:
- * - The iterators do not throw.
+ * single stream. It currently makes an assumption:
  * - The final result (the value returned and not yielded from the iterator)
  *   does not matter; if there is any, it is discarded.
  */
 export class MuxAsyncIterator<T> implements AsyncIterable<T> {
   private iteratorCount = 0;
   private yields: Array<TaggedYieldedValue<T>> = [];
+  // eslint-disable-next-line @typescript-eslint/no-explicit-any
+  private throws: any[] = [];
   private signal: Deferred<void> = deferred();
 
   add(iterator: AsyncIterableIterator<T>): void {
@@ -25,11 +26,15 @@ export class MuxAsyncIterator<T> implements AsyncIterable<T> {
   private async callIteratorNext(
     iterator: AsyncIterableIterator<T>
   ): Promise<void> {
-    const { value, done } = await iterator.next();
-    if (done) {
-      --this.iteratorCount;
-    } else {
-      this.yields.push({ iterator, value });
+    try {
+      const { value, done } = await iterator.next();
+      if (done) {
+        --this.iteratorCount;
+      } else {
+        this.yields.push({ iterator, value });
+      }
+    } catch (e) {
+      this.throws.push(e);
     }
     this.signal.resolve();
   }
@@ -46,6 +51,12 @@ export class MuxAsyncIterator<T> implements AsyncIterable<T> {
         this.callIteratorNext(iterator);
       }
 
+      if (this.throws.length) {
+        for (const e of this.throws) {
+          throw e;
+        }
+        this.throws.length = 0;
+      }
       // Clear the `yields` list and reset the `signal` promise.
       this.yields.length = 0;
       this.signal = deferred();