add exceptions rake task

lib/tasks/exceptions.rake

@@ -0,0 +1,10 @@
+namespace :exceptions do
+  task :list => :environment do
+    exceptions = []
+    ObjectSpace.each_object(Class) do |k|
+      exceptions << k if k.ancestors.include?(Exception)
+    end
+
+    puts exceptions.sort { |a,b| a.to_s <=> b.to_s }.join("\n")
+  end
+end