Require crop name and user for guides

# Why?

 * Hit some nil errors while exploring the 'guides' feature.

# What Changed?

 * Added validations so that users can not create guides unless they have a user, name and crop associated to them.

@simonv3 Could you give this one a good review? I want to make sure I'm not trampling any work you had done.
Does RoR automatically fill in the default strings for errors here?