download: add retries argument

[dabrain34]

Allow multiple attempts to download an asset

Fix #150

[dabrain34]

Confirned with a docker image

[mdimopoulos]

@dabrain34 thanks for the contribution. There is a linter warning in the CI, regarding an exception that is too general. Could you take a look and make it more concrete?

[dabrain34]

I disabled the pylint warning as we need to catch some broad exceptions from the downloader.

[ylatuya]

I disabled the pylint warning as we need to catch some broad exceptions from the downloader.

I think the type of errors that can be fixed with a retry are all urllib.error.URLError

[mdimopoulos]

@dabrain34 there is a mypy error Module has no attribute "URLError". Can you check it out please?
You can run make check before pushing to make sure that all linters pass correctly (ref https://github.com/fluendo/fluster#how-can-i-contribute)