fix network listeners plugin

need to require 'sigar' early so that it blows up during the
definition rather than execution with a LoadError (is this a bug in
ohai core?  this feels like a hack).  also drop hpux and aix
specificity, not sure why that got added.
remove :sigar "platform"

i don't think this platform makes any sense, suspect we need to
remove it everywhere
rescue exceptions thrown collecting data

back out the require 'sigar' hack
propagate Ohai exceptions up the stack
fix specs for exception handling

- setup a class heirarchy of exceptions so i can easily rethrow
  any internal ohai error
- catch all the non-internal errors from running the plugin and
  just warn on those
- convert the argumenterror raises to ohai invalidplugin errors
change base of exceptions to StandardError
rethrow internal ohai errors
Merge pull request #261 from opscode/lcg/fix-network-listeners

Lcg/fix network listeners