Fixes contradictory log messages.

[PartialVolume]

1. The log reported verification errors while also
   showing an entry in the log that said "[SUCCESS] Blanked /dev/...".
   The blanked device message now shows [FAILURE] /dev/... may not be blanked"
   if any verification errors are detected for a specific drive.

2. If a verification error occurred, the error would be correctly
   shown in the GUI and in the logs but the summary table drive status
   would show 'ERASED' not 'FAILED'. Now corrected so that the tables
   drive status field shows 'FAILED' if any verification error is detected.
   Prior to this it was marking the status as 'FAILED' only if the O.S
   detected write errors. In practise most drive errors are detected
   by the write I/O process on syncing but this will now detect errors not
   recognised by the O.S. and found by the verification process. Despite
   this the textual log and GUI correcty reported verification errors.

3. The final log message "Nwipe exited successfully" was checking for
   fatal errors but ignoring non fatal errors despite being reported in
   the log.

The final message now reads
either "Nwipe Exited Succesfully" if no fatal and non fatal errors were
detected. Alternatively it displays ...
"Nwipe exited with errors, check the log & summary table for individual drive status."
if any fatal OR non fatal errors are detected.