Raise an APIRequestFailure exception when security scanner is unavailable

Put worker to sleep for the duration of the default indexing interval
when an APIRequestFailure occurs, when the API request fails due to a
connection error, timeout, or other ambiguous errors, from
analyze_layer or get_layer_data .

util/secscan/analyzer.py

@@ -57,7 +57,7 @@ class LayerAnalyzer(object):
     except AnalyzeLayerRetryException:
       # Something went wrong when trying to analyze the layer, but we should retry, so leave
       # the layer unindexed. Another worker will come along and handle it.
-      pass
+      raise APIRequestFailure
     except MissingParentLayerException:
       # Pass upward, as missing parent is handled in the analyze_recursively method.
       raise
@@ -145,7 +145,7 @@ class LayerAnalyzer(object):
         try:
           layer_data = self._api.get_layer_data(layer, include_vulnerabilities=True)
         except APIRequestFailure:
-          layer_data = None
+          raise
 
         if layer_data is not None:
           # Dispatch events for any detected vulnerabilities