FileBrowser: show errors in messagebox and allow expand 'all'

FileBrowser: show errors in messagebox and allow expand 'all'

If an error is received upon expanding a node, chances are the rest of
the tree is still valid (i.e. opening a partition fails because it
doesn't contain a supported filesystem). Only show an error box for the
user, but don't mask the component in that case. Additionally, disable
the download button.

Also support an archive set to 'all' to expand all children, useful for
initializing a file-restore VM on initial load.

Signed-off-by: Stefan Reiter <s.reiter@proxmox.com>