Better, more informative errors when pull fails (maybe other network operations as well)

[apostasie]

What is the problem you're trying to solve

Servers may misbehave, or return html error pages in certain conditions. Then some images may be broken.

Looking at things like #2676 - it is really hard to tell what is going on - for the user first, and for us reading the tea leaves in bug reports...

Describe the solution you'd like

Suggestion is first to strengthen our user error reporting, and give more informative messages than "snapshotter faceplant, bye".

We should say at least say:

• DNS resolution failed: unable to reach your DNS server
• Network issue: unable to connect to server XYZ
• Server is misbehaving: received 500 / error page
• Image is broken: unknown content type yada
• etc

Furthermore, we should ramp up our testing. We are testing happy path, for the most part, and we should start figuring out what happens when things are not working...

The benefits should be two-fold:

• much better user experience
• easier for maintainers to spot actual issues

Additional context

No response