window/FileBrowser: try reload again when getting a 503 error

window/FileBrowser: try reload again when getting a 503 error

for the file restore, we return a 503 error when we were not finished
mounting a disk in the restore vm, so ignore that error and try again
(up to 10 times) so a file listing now has a "real" timeout of
up to 300 seconds (30s pveproxy timeout * 10) instead of only 30,
which should be enough for most situations.

we also increase the proxy timeout to 60 seconds, since if one has many
disks, all of them will try to load at the same time, but the browser
has a maximum request limit and will stall+queue the remaining ones. so
those will not run into the extjs timeout when we increase it here.

for older backends without the new 503 returning feature, the calls
will still run into a pveproxy timeout anyway.

we also have to reimplement the 'monStoreErrors' functionality to
get a slightly different behaviour:
we disable the default extj loadMask of the treepanel and set it
ourselves. then on 503 we leave it up, and only remove it on success
or error (for non initial loads)

Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>