This is a new system that allows other skills to register as fallbacks to handle general knowledge queries. For now, each skill self-assigns a priority where 0 is the highest and 100 is very low priority. When an intent failure occurs, each fallback gets run until one returns True meaning it found a response to speak to the user. Example usage:

skill-my-fallback/__init__.py:

from mycroft.skills.core import MycroftSkill
class MyFallbackSkilll(MycroftSkill):
    def __init__(self):
        MycroftSkill.__init__(self, name="MyFallbackSkill")

    def initialize(self):
        self.register_fallback(self.handle_fallback, 80)

    def handle_fallback(self, message):
        if 'what is' in message.data['utterance']:
            self.speak_dialog('the answer is always 42')
            return True
        return False

This PR also removes the multi utterance intent fail. It only makes sense to emit an intent_failure regardless of the amount of intents, especially since we only ever receive one utterance anymore.

This PR relies on this PR in skill-wolfram-alpha. Instructions for testing:

git fetch
git checkout feature/fallbacks
mycroft_dir="$(pwd)"
cd /opt/mycroft/skills/skill-wolfram-alpha
git fetch
git checkout feature/fallbacks
cd "$mycroft_dir"
./mycroft.sh start

Then test functionality of Wolfram Alpha for questions like what is a fox (and maybe wolfram skill reloading when /opt/mycroft/skill-wolfram-alpha/__init__.py is edited).