The client-example has a confusing return for the `download` function.

[kairoaraujo]

Description of issue: The client-example has a confusing return for the download function. The return mixes the error and flows handling.

Current behavior: Currently, it returns a bool.

• If a target file is downloaded it returns True
• If there is an exception in the code it returns False
• if a target file is not found it returns True

https://github.com/theupdateframework/python-tuf/blob/c12be4857d5388eb8e3c2e692fa6cce4f95cff35/examples/client_example/client_example.py#L78

Expected behavior: The return code only manages the exceptions. However, there is no use for the current return.

[kairoaraujo]

Thread in Slack channel: https://cloud-native.slack.com/archives/C01GT17AC5D/p1657718611407839

[jku]

I think this may get fixed in #2193