scripts: sd_fusing: handle lsblk errors better

scripts: sd_fusing: handle lsblk errors better

This makes the error handling a bit more consistent with the rest of the
script by logging an error and doing a sys.exit(1).

Returning an error by returning None would mean that it would need to be
handled elsewhere. However this does not happen, resulting in a
confusing runtime error

    TypeError: can only concatenate str (not "NoneType") to str

as well as warnings in some editors.

Change-Id: I6b3f382e93846c3165d1a8221e2faf7dbf295a86