Remove ignoreHeaders argument from parseFilter(s) and move the logic to the UI

[sebastian]

Background

After #2658, trev suggested - and I agree - to also get rid of the ignoreHeaders argument in favor of slightly more complex data structures, leaving the handling of that case to the UI code.

What to change

• Replace the error string by an error object, with a toString() method, and a type property in order to distinguish unexpected filter list headers from other reasons a filter is considered invalid.
• Remove the ignoreHeaders argument from parseFilter() and parseFilters().
• Make parseFilters() return an array of errors like {filters: [..], errors: [..]} instead of bailing out on the first validation error.
• Adjust UI code and tests.

Hints for testers

Make sure that filter validation still works as it did before. See 2658#Hintsfortesters for detailed instructions.

Also pay particular attention to the handling of filter list headers. For example, if you enter [Adblock Plus 2.0] in raw edit mode on the options page, that line should simply be stripped. However, if you attempt to add that string using the single-line text field at the top, you should get an error message.

[sebastian]

​https://hg.adblockplus.org/adblockpluschrome/rev/35b96cf12609