debian: Do not raise when a component cannot be found for a suite

All debian suites do not necessarily have the same set of components.

So prefer to log that a component is missing for a suite instead of
raising an excption that will stop the listing.

swh/lister/debian/lister.py

@@ -134,19 +134,17 @@ class DebianLister(Lister[DebianListerState, DebianPageType]):
             response = requests.get(url, stream=True)
             logging.debug("Fetched URL: %s, status code: %s", url, response.status_code)
             if response.status_code == 200:
+                decompressor = decompressors.get(compression)
+                if decompressor:
+                    data = decompressor(response.raw).readlines()
+                else:
+                    data = response.raw.readlines()
                 break
         else:
-            raise Exception(
-                "Could not retrieve sources index for %s/%s", suite, component
-            )
+            data = ""
+            logger.debug("Could not retrieve sources index for %s/%s", suite, component)
 
-        decompressor = decompressors.get(compression)
-        if decompressor:
-            data = decompressor(response.raw)
-        else:
-            data = response.raw
-
-        return Sources.iter_paragraphs(data.readlines())
+        return Sources.iter_paragraphs(data)
 
     def get_pages(self) -> Iterator[DebianPageType]:
         """Return an iterator on parsed debian package Sources files, one per combination

swh/lister/debian/tests/test_lister.py

@@ -36,7 +36,7 @@ from swh.scheduler.interface import SchedulerInterface
 
 _mirror_url = "http://deb.debian.org/debian"
 _suites = ["stretch", "buster", "bullseye"]
-_components = ["main"]
+_components = ["main", "foo"]
 
 SourcesText = str
 
@@ -59,6 +59,7 @@ def _init_test(
     debian_sources: Dict[Suite, SourcesText],
     requests_mock,
 ) -> Tuple[DebianLister, DebianSuitePkgSrcInfo]:
+
     lister = DebianLister(
         scheduler=swh_scheduler,
         mirror_url=_mirror_url,
@@ -79,6 +80,9 @@ def _init_test(
             else:
                 requests_mock.get(idx_url, text=sources)
 
+        for idx_url, _ in lister.debian_index_urls(suite, _components[1]):
+            requests_mock.get(idx_url, status_code=404)
+
     return lister, suite_pkg_info
 
 
@@ -198,7 +202,7 @@ def test_lister_debian_updated_packages(
             lister_previous_state=lister_previous_state,
         )
 
-        assert stats.pages == len(sources)
+        assert stats.pages == len(sources) * len(_components)
         assert stats.origins == len(origin_urls)
 
         lister_previous_state = lister.state.package_versions