Assert no spaces in crate features JSON

[ahmedaabouzied]

Spaces in the crate_features JSON value can cause errors when running kcov.

The json.load() method used to load the JSON file will raise an error if it's not valid JSON as mentioned on the python json module docs

If the data being deserialized is not a valid JSON document, a JSONDecodeError will be raised.

Fixes: #74

[ahmedaabouzied]

@lauralt , Done