host-ctr image pull: differentiate transient and permanent errors and handle them differently

[etungsten]

Either way, I think to make this reliable we'll also want to further designate errors to be handled to determine transient failures (where possible) from permanent failures (eg: user specified a bad image). This can be a fast follow item IMO. I think this already gets us to a better standing in terms of transient errors.

Originally posted by @jahkeup #433 (review)

[etungsten]

Hopefully somewhere down the line we can leverage what's being discussed here https://blog.golang.org/go1.13-errors

[jahkeup]

We do have some of these provided by the use of the github.com/pkg/errors package in the downstream project - so we may be able to dig in. However, this may not be help if there's not predefined errors or distinguishable errors used - which on cursory glance appears to be the case at the API surface area; it is possible the api surface area is bubbling up well defined errors we can handle though.