test/TEST-14-IMSM: detect failure to compose the test setup root

test/TEST-14-IMSM: detect failure to compose the test setup root

The dracut-root-block-created line should not be created if we fail to copy
in the required files to sysroot. Let's turn on -e to trap failures and
poweroff on them, like some other tests do.

Also remove the &&. Not only it is unnecessary with -e, but defeats it.
From bash(1):

  The shell does not exit if the command that fails is [...] part of any
  command executed in a && or || list except the command following the
  final && or || [...]