Obscure error when vars: data_diff: not found

[dlawin]

We should output a more descriptive error when dbt_project.yml vars: data_diff: is not found

Current:
[12:52:44] ERROR - 'NoneType' object has no attribute 'get'

Ideal:
ERROR - Did not find a dbt_project.yml variable named "data_diff"

We can reuse this utility method

data-diff/data_diff/utils.py

Line 82 in e09a45b

def get_from_dict_with_raise(dictionary: Dict, key: str, error_message: str):