[WIP] dvc: wrap checkout error as download error

[Suor]

This will happen on missing caches and similar to download error in its
effect.

We might want to think of making missing caches an error. That is, however, not an easy thing to do since we will need to postpone/collect those errors in fetch and turn into a warning in status.

[Suor]

So the issue is when we pull an imported thing and that doesn't exist (repo or its cache was deleted) then we get CheckoutError instead of DownloadError.

We can wrap to fix it, however, the core issue is that DataCloud.pull() is silently ok with missing files. So we might want to change that instead.

[efiop]

@Suor So do we need this PR?

[Suor]

Do we worry about a wrong error message? If we do then we should. The message now is:

Checkout failed for following targets:
     <some file name>
Did you forget to fetch?

While that might be caused by an output missing in a cache and silently skipped during fetch or that someone removed/moved source repo cache folder.

This PR only shows the problem via failing test, but doesn't fix it. We might add a solution on top.

[efiop]

@Suor Didn't mean to push, just was checking what is the status here. Ok, let's include it into the next sprint then to fix it.

[Suor]

Looks like this is low priority for everyone including myself. Closing this for now, will handle when we run into it or with some related refactoring later.