update_oai-pmh robust against metadata prefix lookup failures

Motivation:

The ListMetadataFormats call can fail.  Currently, this causes the enire
script to fail.

Modification:

Catch the exception and report a failure.

Result:

A metadata prefix lookup failure is now limited to a single
OAI-PMH endpoint

scripts/update_oai-pmh.rb

@@ -346,7 +346,12 @@ def query_oai_pmh_endpoint(endpoint)
         return status, [], {}, 0, {}
     end
 
-    dc_prefix = metadata_prefix_of(endpoint, 'http://www.openarchives.org/OAI/2.0/oai_dc/')
+    begin
+        dc_prefix = metadata_prefix_of(endpoint, 'http://www.openarchives.org/OAI/2.0/oai_dc/')
+    rescue StandardError => e
+        print_with_time("Error: metadata_prefix_of failed: #{e.message}")
+        return "Error", [], {}, 0, {}
+    end
 
     set_names = list_sets(endpoint)