Garbage colleciton fix

src/manager.js

@@ -169,8 +169,12 @@ export class ZimManager {
 				pending = new ZimReader(file, {clusterTTL: this.#clusterTTL}).open();
 				this.#opening.set(file, pending);
 			}
-			const reader = await pending;
-			this.#opening.delete(file);
+			let reader;
+			try {
+				reader = await pending;
+			} finally {
+				this.#opening.delete(file);
+			}
 			entry = this.#readers.get(file) ?? {reader};
 			this.#readers.set(file, entry);
 		}