Add link to docs in error message about not providing a callback to .exec().  Also explicitly notice when something totally weird like an array or number or stream is passed in to .exec() instead of a valid callback.
Add link to docs in error message about not providing a callback to .exec().  Also explicitly notice when something totally weird like an array or number or stream is passed in to .exec() instead of a valid callback.

luislobo/waterline,balderdashy/waterline