backups: remove validation mount point after testing it

this also moves out the attempt validation logic from mounts code
into volumes. mounts.tryAddMount is also used in backup code

src/scripts/addmount.sh

@@ -28,7 +28,7 @@ mount_file="/etc/systemd/system/${mount_filename}"
 
 # cleanup any previous mount of same name (after midway box crash?)
 if systemctl -q is-active "${mount_filename}"; then
-    echo "Previous mount active, unmounting"
+    echo "Previous mount ${mount_filename} active, unmounting"
     # unmounting can fail if a user is cd'ed into the directory. if we go ahead and mount anyway, systemd says "active" because it's referring to the previous mount config
     if ! systemctl stop "${mount_filename}"; then
         echo "Failed to unmount"
@@ -45,7 +45,7 @@ if ! timeout "${timeout}" systemctl enable --now "${mount_filename}"; then
     exit 3
 fi
 
-echo "Mount succeeded"
+echo "Mount ${mount_filename} succeeded"
 
 # this has to be done post-mount because permissions come from the underlying mount file system and not the mount point
 chmod 777 "${where}"