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.
Yeah. You would find the error message in
`object.errors.messages[:fieldname]`

Sent from a mobile device. Please excuse my brevity.
On Aug 24, 2014 12:35 PM, "Simon" <notifications@github.com> wrote:

> In app/models/guide.rb:
>
> >    field :name
> >    field :overview
> > -
> > +  validates_presence_of :user, :crop, :name
>
> Does RoR automatically fill in the default strings for errors here?
>
> —
> Reply to this email directly or view it on GitHub
> <https://github.com/FarmBot/OpenFarm/pull/97/files#r16636933>.
>