Shouldn't fail when path is not available

When I specify multiple mounts, and let's say the first one doesn't exist, it is not trying to cache the other paths.

I worked around it by splitting the cache tasks up (load-a-cache, restore-a-cache, load-b-cache, restore-b-cache, etc...)

I think it would be nice if it would still cache other paths that ARE available.

Start by tracing how multiple mounts are processed in the cache plugin, focusing on what happens when the first path is unavailable. Done means an unavailable path does not prevent other available paths from being cached; verify the behavior with a multiple-mount case.

Written by the indexing model from the issue text.