volume_manager: Move bind/mount failure from ERROR to WARNING

* Prevents users from seeing this message on FBE devices:
  `failed to bind mount /mnt/staging/emulated/media/0 to /storage/emulated`

* FDE users already have the workaround in line 75.

Change-Id: I47d1aaf1a5a85a46fd52dc924a91b378cd800b2d