fix error handling.

Turns out without the try: finally statement the clean up step gets skipped.
https://github.com/flutter/flutter/issues/68144#issuecomment-712299364

Change-Id: I47cb84bea02a386499c75674cef3b7dc47376d78
Reviewed-on: https://flutter-review.googlesource.com/c/recipes/+/7780
Commit-Queue: Godofredo Contreras <godofredoc@google.com>
Commit-Queue: Weiyu Huang <weiyuhuang@google.com>
Reviewed-by: Godofredo Contreras <godofredoc@google.com>