catch and ignore failure to load chef

if chef version is 11.8.2 and it pins ohai to <~ 6.0, then if we
install ohai 7, the chef plugin will attempt to load chef up and
then will puke hard since ohai 7 is already loaded.  this just skips
chef version detection if that dependency resolution problem occurs.
add comment on why rescue'ing
Merge pull request #258 from opscode/lcg/catch-chef-loaderror

catch and ignore failure to load chef