nsupdate_plugin: Exception logging now includes error output from nsupda...

...te command

Bug 1089697
Bugzilla link https://bugzilla.redhat.com/show_bug.cgi?id=1089697
When troubleshooting DNS record addition/removal errors, it is helpful to have the error output from the nsupdate command. Specifically in scenarios where record changes only fail <1% of the time.