[css-nesting-1] Need to specify error-handling when writing selectors via OM

[tabatkins]

I need to specify what happens when you insert a nested style rule via the OM with a selector that violates the requirement to not start with an identifier or function, or modify the .selectorText of an existing rule to violate it.

I think the .selectorText update rule should do the check and, like if you try to set it to anything else that's not a valid selector, just do nothing.

For rule insertion, unsure whether we should silently drop such rules or raise an error.

[Loirooriol]

https://drafts.csswg.org/cssom/#insert-a-css-rule throws in case of error:

2. If index is greater than length, then throw an IndexSizeError exception.

4. If new rule is a syntax error, throw a SyntaxError exception.
5. If new rule cannot be inserted into list at the zero-index position index due to constraints specified by CSS, then throw a HierarchyRequestError exception.
6. If new rule is an @namespace at-rule, and list contains anything other than @import at-rules, and @namespace at-rules, throw an InvalidStateError exception.

So I think it should be the same for nesting.

[tabatkins]

Retagging to CSSOM, as that's where the algorithm that needs modification lives. It looks like it should throw a SyntaxError (thanks, @Loirooriol).