Errors.add should throw on invalid calls

To simplify rules debugging, prevent dumb mistakes and crashes in error reporters.
If you'll look at https://github.com/zxqfox/node-jscs/blob/hotfix/prevent-crashes-in-error-reporters/lib/string-checker.js#L89-L103 you'll see a catching parse error and puting `err.lineNumber` to `errors.add`. So if `lineNumber` is empty it will throw.