Use warn() when logging an error for setup_mount_destination()

setup_mount_destination() does not always set |errno| before
returning. Also, setup_mount_destination() now has pwarn()s
for each failure path.

Bug: None
Test: unit tests still pass
Change-Id: Ie603cffb1b8ea28a5f92a4d790b92e281ff2c6a0
1 file changed