Display failure from wipefs as an error not a warning (#688882)

When wipefs command fails this means that in the display of the
operation results tree:

 1) The command is displayed with a time and a stop sign, rather than no
    time and a warning sign;

 2) There is no report of any warnings or failures at the top level
    summary.

(The immedately following "clear primary signatures" step will clear all
the file system signatures which wipefs failed to do.  Should this fail
too it will report errors which will fail the whole operation).

Bug #688882 - Improve clearing of file system signatures