convert ntpq "when" output to second when necessary

ntpq program can produce pretty value for "when" when last packet
arrived for a long time, such as Xm (for X minutes), Xh (for X hours),
Xd (for X days). This commit converts all that pretty outputs to seconds as
diamond needs values to be float numbers.
put more infromation in exception to help indicating problem source

if user use sentry to log exception out, he only need to look into sentry
to know what went wrong without checking log file
Merge pull request #610 from hvnsweeting/enhance-exception-info

put more infromation in exception to help indicating problem source
Merge pull request #609 from hvnsweeting/handle-ntp-pretty-print

convert ntpq "when" output to second when necessary