Return a better error for volume name conflicts

When you try and create a new volume with the name of a volume
that already exists, you presently get a thoroughly unhelpful
error from `mkdir` as the volume attempts to create the
directory it will be mounted at. An EEXIST out of mkdir is not
particularly helpful to Podman users - it doesn't explain that
the name is already taken by another volume.

The solution here is potentially racy as the runtime is not
locked, so someone else could take the name while we're still
getting things set up, but that's a narrow timing window, and we
will still return an error - just an error that's not as good as
this one.

Signed-off-by: Matthew Heon <matthew.heon@pm.me>

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