Don't raise an exception when a DB is not yet available

For example, the first time a task is created.

bugbug/db.py

@@ -85,7 +85,7 @@ def download(path, force=False, support_files_too=False):
 
         except requests.exceptions.HTTPError:
             print(f"{url} is not yet available to download")
-            raise
+            return
 
     extract_file(zst_path)
 

tests/test_db.py

@@ -193,8 +193,8 @@ def test_download_missing(tmp_path):
         responses.GET, url, status=404, body=requests.exceptions.HTTPError("HTTP error")
     )
 
-    with pytest.raises(requests.exceptions.HTTPError):
-        db.download(db_path)
+    db.download(db_path)
+    assert not os.path.exists(db_path)
 
     with pytest.raises(Exception, match="Last-Modified is not available"):
         db.last_modified(db_path)