first stage mount: abort when failed to parse fstab in device-tree

The client of FirstStageMount class should check the existence of fstab
in device-tree prior to using it. So raising a FATAL error inside
FirstStageMount when failed to parsing the fstab, in order to expose
more accurate error messages.

Also fixing a comment in fs_mgr, where it might happen in either
non-A/B or A/B.

Bug: 69102431
Test: boot sailfish
Change-Id: Ifb525beaa3423b118644778bfe0f84bff9465303