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 ====
MycroftAI#2120

mycroft/skills/mycroft_skill/mycroft_skill.py

@@ -686,7 +686,7 @@ def find_resource(self, res_name, res_dirname=None):
         return result
 
     def _find_resource(self, res_name, lang, res_dirname=None):
-        """Find resource by name, lang and dir
+        """Finds a resource by name, lang and dir
         """
         if res_dirname:
             # Try the old translated directory (dialog/vocab/regex)