Fix errors not being logged for plugin.Gather

[nickscript0]

Currently if plugin.Gather returns an error it does not show up in the telegraf log. This PR adds logging to show an error in both crankParallel and crankSeparate cases.

For example a misconfigured plugin as follows currently show no errors:

[mysql]
servers = ["notlocalhost"]

After this fix it shows an error as expected:

./telegraf -config /etc/opt/telegraf/telegraf.conf
2015/08/22 20:48:07 Starting Telegraf
2015/08/22 20:48:07 Loaded outputs: influxdb
2015/08/22 20:48:07 Loaded plugins: cpu mysql
2015/08/22 20:48:07 Tags enabled: dc=test1 host=a11790
2015/08/22 20:48:07 Error in plugins: Invalid DSN: Missing the slash separating the database name

[sparrc]

Please add plugin.name to the debug message, something like

log.Printf("Error in plugin [%s]: %s", plugin.name, err)

[sparrc]

Thanks @nickscript0! Please add your change to the CHANGELOG under features

You'll also need to sign the CLA, I'll send details shortly

[sparrc]

Oops, nevermind, I see that you've already signed it, thanks!

I'll merge when you add the comments I made, thanks much!

[nickscript0]

Addressed your comments: added plugin name and updated CHANGELOG. Should be good to merge now.

[sparrc]

Thanks @nickscript0! These changes have been rebased into master