Flatten: improve error handling

Flatten provides also information about invalid manifests, so we cannot
handle output of flatten as server error only.

Now OMPS check exception raised from flatten to provide more accurate
error.

Signed-off-by: Martin Bašti <mbasti@redhat.com>

omps/api/v1/push.py

@@ -25,7 +25,7 @@
     OMPSUploadedFileError,
     OMPSExpectedFileError,
     QuayPackageNotFound,
-    QuayCourierError,
+    raise_for_courier_exception,
 )
 from omps.greenwave import GREENWAVE
 from omps.koji_util import KOJI
@@ -178,9 +178,7 @@ def _flatten_manifest_structure(source_dir, dest_dir):
     try:
         flatten(source_dir, dest_dir)
     except OpCourierError as e:
-        raise QuayCourierError(
-            'Failed to flatten manifest directory: {}'.format(e)
-        )
+        raise_for_courier_exception(e)
 
     if not os.listdir(dest_dir):
         # if dest dir is empty, it means that flatten did noop and source dir