Improve robustness of the remove-latest cli command

[dennisvang]

To roll back a release, we can call tufup targets remove-latest.

This takes care of the following:

1. remove the target entries for the latest release from targets.json
2. remove the actual target files from the repository's targets dir
3. re-sign targets.json, snapshot.json, and timestamp.json

In some cases, we may have already removed the target files, or we are reconstructing a repo just to update the metadata, and do not want to download the actual target files.

However, if the target files are not present, the remove-latest command fails with a FileNotFoundError.

A workaround is to create an empty "dummy" file at the specified path, but that is inconvenient.

To make the command more robust, we should catch the FileNotFoundError, show a warning, and continue with the metadata update.