lister/tasks: Standardize return statements

The following commit adapts the return statements from both lister and their
associated tasks. This standardizes on what other modules (e.g. both dvcs and
package loaders) do.

swh/lister/debian/lister.py

@@ -216,12 +216,14 @@ class DebianLister(ListerHttpTransport, ListerBase):
                            .one_or_none()
 
         if not distribution:
-            raise ValueError("Distribution %s is not registered" %
-                             self.distribution)
+            logger.error("Distribution %s is not registered" %
+                         self.distribution)
+            return {'status': 'failed'}
 
         if not distribution.type == 'deb':
-            raise ValueError("Distribution %s is not a Debian derivative" %
-                             distribution)
+            logger.error("Distribution %s is not a Debian derivative" %
+                         distribution)
+            return {'status': 'failed'}
 
         date = self.date
 
@@ -250,4 +252,4 @@ class DebianLister(ListerHttpTransport, ListerBase):
 
         self.db_session.commit()
 
-        return True
+        return {'status': 'eventful'}

swh/lister/debian/tasks.py

@@ -10,7 +10,7 @@ from .lister import DebianLister
 @shared_task(name=__name__ + '.DebianListerTask')
 def list_debian_distribution(distribution, **lister_args):
     '''List a Debian distribution'''
-    DebianLister(distribution=distribution, **lister_args).run()
+    return DebianLister(distribution=distribution, **lister_args).run()
 
 
 @shared_task(name=__name__ + '.ping')