nessus: formatting fix.
NessusXMLRPC: added getErrors method.

Method to get the error events for a specific job, because Nessus
needs a separate call for that.
nessus: report email contains errors encountered during scans.

Previously the script ignored all errors encountered by Nessus during
scan(s), and the only indication of something problematic was that the
report was empty.
nessus: missing debug option in the command line is not fatal.