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

I passed a bad value to ntpdcollector to test this, it now displays:

````
  File "/home/hvn/python2/local/lib/python2.7/site-packages/diamond/collector.py", line 429, in _run
    self.collect()
  File "/home/hvn/Github/Diamond/src/collectors/ntpd/ntpd.py", line 113, in collect
    self.publish(stat, val)
  File "/home/hvn/python2/local/lib/python2.7/site-packages/diamond/collector.py", line 343, in publish
    metric_type=metric_type, ttl=ttl)
  File "/home/hvn/python2/local/lib/python2.7/site-packages/diamond/metric.py", line 56, in __init__
    "Metric %r: %s") % (path, e))
DiamondException: Invalid value when creating new Metric 'servers.hvnwork.ntpd.when': invalid literal for float(): 1y
````