Errors.add should throw on invalid calls

To simplify rules debugging, prevent dumb mistakes and crashes in error reporters.
>> And I guess you should specify a column as well?
> Column is optional atm so I would like to know why. I should fix that if it does only because of duck typing.

Column is not optional. You must pass line and column as two separate arguments or `{line: Number, column: Number}` as a single argument.