BUGFIX: drop data when ntpd returns "-" for "when" field
Log path and value passed to Metric() if exception raised

this helps indicating source of exception from which data collected
by collector
Merge pull request #586 from hvnsweeting/fix-ntpd-edge-case

BUGFIX: drop data when ntpd return - for when field
Merge pull request #599 from hvnsweeting/hvn-enhance-log

Log path and value passed to Metric() if exception raised