pull: download data from imported stages

[ghost]

Fix #2423

TODO:

• Write a test for DownloadError
• Wrap dependency.repo.fetch to always return DownloadError (what about a git clone error? what about a find_out_by_relpath error? This will continue halting the repo.fetch operation)

[ghost]

@efiop , would you mind taking another look?

[efiop]

For the record: discussed during our meeting.

[efiop]

Alright, let's go with this as a temporary solution. We will revisit this with a refactor soon.

[Suor]

My review point

This still looks wrong. If remote is not configured and there are imports than the error will be silently ignored.

was lost along the way. I've created a new issue for it - #2579