remount: Remove errno test & improve messaging of fs_mgr_overlayfs_mount_all

* remount don't check errno after calling fs_mgr_overlayfs_mount_all()
  as we don't report error status through errno anymore.
* fs_mgr_overlayfs_mount_all() returns false if any failure.
* fs_mgr_overlayfs_mount_all() returns true if no overlayfs to mount or
  all overlayfs are either already mounted or mounted successfully.

Bug: 241179247
Bug: 248295731
Test: Treehugger
Change-Id: Ia9c7ac686f6538a9f5da7efc4cda6f28aff056f6