example-client: better error handling

The example client has two conflicting goals:

• provide “production-like” error messages: don’t dump a 300 line stack trace if the network is down
• be a useful debugging tool

For an example of the second I modified the example client to use some real world repositories: the error messages I got were mostly useless – I needed to remove the error handling in the example app to see what the errors were.

So I suggest a couple of things:

• DeserializationError needs an actual error message when it’s raised – failure to deserialize local root metadata currently looks like following, note the missing error message on last line (this can be reproduced by just modifying the local root.json file so it is invalid). This should now be fixed with #1876

  $ ./client_example.py  download x
  Found trusted root in /home/jku/.local/share/python-tuf-client-example
  Failed to download target x: 
  

• Maybe there should be a way to see the actual stack trace, and that should be documented in the error message. So a hypothetical normal failure would look like:

  $ ./client_example.py download x
  Found trusted root in /home/jku/.local/share/python-tuf-client-example
  Failed to download target x: Failed to transmogrify metadata
    (For detailed traceback use "-v")
  

  and then using -v would raise e instead of handling it? This would be a small change in client_example itself

[ameypd]

I would like to take this up as my first PR.

[ameypd]

@jku @lukpueh I have passed verbose as parameter to download function and if the verbose is anything other than default I am printing the stack trace.