Add exception information to the InternalErrorResponse stringification

Author: p

lib/datadog/core/transport/response.rb

@@ -55,6 +55,10 @@ def internal_error?
           true
         end
 
+        def to_s
+          "#{super}, error_type:#{error.class} error:#{error}"
+        end
+
         def inspect
           "#{super}, error_type:#{error.class} error:#{error}"
         end

spec/datadog/core/transport/response_spec.rb

@@ -68,4 +68,10 @@
 
     it { is_expected.to be true }
   end
+
+  describe '#to_s' do
+    it 'includes the causing exception' do
+      expect(response.to_s).to match(/StandardError/)
+    end
+  end
 end