added handling for data retrieval errors

app/services/data/load_element_data.rb

@@ -65,6 +65,11 @@ class LoadElementData
       if data_service.success?
         combined_data.concat(data_service.data)
       else
+        #create error entries
+        error_entries = stream_elements.map do |e|
+          {id: e.id, type: 'error', values: nil}
+        end
+        combined_data.concat error_entries
         add_warning("unable to retrieve data for #{stream.path}")
       end
     end

spec/services/data/load_element_data_spec.rb

@@ -86,7 +86,8 @@ RSpec.describe 'LoadElementData' do
       service.run([@elem0, @elem3],0,100)
       expect(service.warnings.length).to eq 1
       expect(service.data).to eq [
-        {id: @elem0.id, values: 'mock0'}
+        {id: @elem0.id, values: 'mock0'},
+        {id: @elem3.id, type: 'error', values: nil}
       ]
       expect(@mock_stream_service.run_count).to eq 2
     end