Fix recovery logic in update-oai-pmh

The recovery logic when faced with an error querying the identifiers is
currently broken, as it returns nil instead of a hash.

scripts/update_oai-pmh.rb

@@ -222,6 +222,8 @@ def query_oai_pmh_endpoint(endpoint)
         total_count, set_counts = count_identifiers(endpoint, dc_prefix)
     rescue StandardError => e
         puts "    Error: ListIdentifiers failed #{e.message}"
+        total_count = 0
+        set_counts = {}
     end
 
     return status, set_names, total_count, set_counts