Detect the absence of the default fstab

This is a follow-up of I828ce999be6d786bf46dd5655dfda81d046906ab. The
change introduced a behavioral change that fstab is read twice: before
root is changed to /first_stage_ramdisk, and once again after that.
Previously, that happend only after the root is switched. That change
caused a problem when there is no fstab in DT and fstab is provided via
a file. The fstab file has been at
/first_stage_ramdisk/fstab.<hardware> because that file was supposed to
be read after the root switch.

With the change, init fails to read the fstab during the first attempt
because there is no /fstab.<hardware> at the moment. Here comes the
problem. Although it failed to read fstab, DoCreateService() is invoked
because ReadFirstStageFstab() doesn't report the failure; it returns an
empty fstab object. As a result, DoCreateDevices() is called but it
doesn't create the dm linear device because it couldn't find an fstab
entry having `logical` option.

Then after /first_stage_ramdisk becomes the root, the fstab file is
correctly read. But since the prior run of DoCreateDevices() is recorded
as 'done', init doesn't try to do that again; dm linear device is never
created. Then we fail to mount any of the logical partitions.

This change fixes the problem by modifying ReadFirstStageFstab()
function so that the failure is correctly reported back to the caller.
When it fails, DoCreateDevices() is not called.

Bug: N/A
Test: Watch TH
Change-Id: Idf2dbc6c0fb6c311ab3f5ff1f28315f7daa2b4ce