Put exceptions on the wire as a hash

This should address issues we were seeing with published exceptions
not having any real data.

lib/dat/science/result.rb

@@ -37,11 +37,16 @@ module Dat
       def payload
         {
           :duration  => duration,
-          :exception => exception,
+          :exception => serialize_exception(exception),
           :value     => experiment.clean(value)
         }
       end
 
+      def serialize_exception(exception)
+        return nil unless exception
+        { :class => exception.class.name, :message => exception.message }
+      end
+
       def raised?
         !!exception
       end

test/dat_science_experiment_test.rb

@@ -257,4 +257,22 @@ class DatScienceExperimentTest < MiniTest::Unit::TestCase
     refute_nil payload[:control][:exception]
     refute_nil payload[:candidate][:exception]
   end
+
+  def test_publishes_exception_data
+    e = Experiment.new "foo"
+    e.control { raise "foo" }
+    e.candidate { raise "bar" }
+
+    assert_raises RuntimeError do
+      e.run
+    end
+
+    event, payload = Experiment.published.first
+    refute_nil event
+    refute_nil payload
+
+    assert_equal :mismatch, event
+    assert_equal({ :message => 'foo', :class => 'RuntimeError' }, payload[:control][:exception])
+    assert_equal({ :message => 'bar', :class => 'RuntimeError' }, payload[:candidate][:exception])
+  end
 end