Log all 'severe errors', 'errors' and 'warnings' to the seprarate
dsmci.errors.log logfile, which an admin can further use to analyze an
error situation.

The file is rewritten on every run and contains the same information
which is later on summarized in the stats report (which counts the
severe errors, errrors an warnings).

Signed-off-by: Salvatore Bonaccorso <bonaccos@ee.ethz.ch>