Check that all keys in the specified key field are valid

[jjnesbitt]

Once multinet-app/multinet-client#54 is merged, users will be able to specify their own key columns, but currently there's nothing in place to verify that the keys in the specified key column conform to the rules that ArangoDB places on keys.

[waxlamp]

This is pretty high priority, since without this we can leave the system in a broken state.