Support new filter validation errors from adblockpluscore 0.5

Background / User story

We wanted to upgrade the adblockpluscore dependency to 0.5 in #929 (closed), but decided to skip it and include that upgrade as part of #929 (closed), where we upgrade the webext-sdk dependency, which brings with it adblockpluscore 0.5. Nevertheless, we need to add error messages corresponding to new filter validation errors that it introduces.

What to change

• Design: N/A
• Research: N/A
• Spec: eyeo/specs/spec!384
• Legal: N/A
• Development: Add support for "filter_url_not_specific_enough" and "filter_elemhide_not_specific_enough" filter validation errors.

Hints for testers

If a custom filter is invalid, because it includes an unknown option, the "filter_unknown_option" error message should be shown. However, if a known option that requires a value is used (e.g. "rewrite"), but no value is given, the "filter_invalid_option" error message should be shown instead.

Hints for translators

• Added strings:
  • filter_elemhide_not_specific_enough
  • filter_url_not_specific_enough
• Updated strings:
  • filter_slow