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 .

workers/securityworker.py

@@ -10,7 +10,7 @@ from workers.worker import Worker
 from data.database import UseThenDisconnect
 from data.model.image import (get_images_eligible_for_scan, get_image_pk_field,
                               get_max_id_for_sec_scan, get_min_id_for_sec_scan)
-from util.secscan.api import SecurityConfigValidator
+from util.secscan.api import SecurityConfigValidator, APIRequestFailure
 from util.secscan.analyzer import LayerAnalyzer, PreemptedException
 from util.migrate.allocator import yield_random_entries
 from endpoints.v2 import v2_bp
@@ -73,6 +73,8 @@ class SecurityWorker(Worker):
         except PreemptedException:
           logger.info('Another worker pre-empted us for layer: %s', candidate.id)
           abt.set()
+        except APIRequestFailure:
+          time.sleep(DEFAULT_INDEXING_INTERVAL)
 
         unscanned_images_gauge.Set(num_remaining)