fs_mrg: clean up multi-type mount_all

Move the code that attempts to mount alternative fstab entries
into its own function.
Clarify return codes.
Suggest wipe via recovery in error messages.

Bug: 15747366
Change-Id: I3634477cd4d1d73f974f3e906c53285f0d9e0eac
Signed-off-by: JP Abgrall <jpa@google.com>