kpm restore should not throw when the user's machine is configured with a package source that does not exist

[PradeepKadubandi]

When I add a package source that does not exist, kpm restore bails on any project and throws ... it should ignore and use other sources instead ...

[Eilon]

I think this is covered by this comment in another issue: #591 (comment)

We do not want to silently ignore missing feeds - that's important information to a developer. However, we will allow ignoring the failures.

[davidfowl]

Yah, it's a dupe