Accept an async listener in the types and normalise any rejection reason

src/error-from.ts

@@ -0,0 +1,10 @@
+/** Whatever was thrown or rejected, as an Error. `String()` throws on some values; this cannot. */
+export function errorFrom(reason: unknown): Error {
+	if (reason instanceof Error) return reason;
+
+	try {
+		return new Error(String(reason));
+	} catch {
+		return new Error('A thrown value that cannot be converted to a string');
+	}
+}