Improve default error handling on collector

More details see:
https://bugs.launchpad.net/cloudkitty/+bug/1529034

Change-Id: I20ba915175bce5dcda72fbfd449770c72b91fce0
Closes-Bug: #1529034

cloudkitty/collector/__init__.py

@@ -73,6 +73,7 @@ class NoDataCollected(Exception):
 
 @six.add_metaclass(abc.ABCMeta)
 class BaseCollector(object):
+    collector_name = None
     dependencies = []
 
     def __init__(self, transformers, **kwargs):
@@ -111,6 +112,8 @@ class BaseCollector(object):
         trans_resource = 'get_'
         trans_resource += resource.replace('.', '_')
         if not hasattr(self, trans_resource):
-            return None
+            raise NotImplementedError(
+                "No method found in collector '%s' for resource '%s'."
+                % (self.collector_name, resource))
         func = getattr(self, trans_resource)
         return func(start, end, project_id, q_filter)

cloudkitty/collector/meta.py

@@ -82,3 +82,7 @@ class MetaCollector(collector.BaseCollector):
         func = self.map_retrieve(trans_resource, res_collector)
         if func is not None:
             return func(start, end, project_id, q_filter)
+        raise NotImplementedError(
+            "No method found in collector '%s' for resource '%s'."
+            % (res_collector.collector_name if res_collector else '',
+               resource))

cloudkitty/orchestrator.py

@@ -156,11 +156,10 @@ class Worker(BaseWorker):
                                             start_timestamp,
                                             next_timestamp,
                                             self._tenant_id)
-
-        timed_data = [{'period': {'begin': start_timestamp,
-                                  'end': next_timestamp},
-                      'usage': raw_data}]
-        return timed_data
+        if raw_data:
+            return [{'period': {'begin': start_timestamp,
+                                'end': next_timestamp},
+                     'usage': raw_data}]
 
     def check_state(self):
         timestamp = self._storage.get_state(self._tenant_id)