Fallback to 'en-us' when resource fails
Fallback to 'en-us' when resource fails

When mycroft_skill.find_resource fails to load a resource for self.lang
fall back to lang 'en-us'
as most skills/resources are available in english by default.

==== Fixed Issues ====
#2120

forslund/mycroft-core,forslund/mycroft-core,MycroftAI/mycroft-core,MycroftAI/mycroft-core