storageServer: fix connect to detect mount errors

Raise the appropriate exceptions in case of mount error or
permissions error to mount point.
Also, exit connect in case of mount failure to avoid using
uncreated mount.

Change-Id: I6bcb9774a8e6bd0f6bbfd8768181158a198f4ec7
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=883877
Signed-off-by: Yeela Kaplan <ykaplan@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/12042
Reviewed-by: Ayal Baron <abaron@redhat.com>