Allow `knife cookbook site share` to omit category

This adds a feature for knife to lookup the category on an existing cookbook. This enables uploading new versions of existing cookbooks with much less pain.
Adjust banner, error messages

Per feedback from @sersut, distinguish between excption and simply Supermarket error or cookbook not found.
Merge pull request #2203 from martinb3/master

Allow `knife cookbook site share` to omit category