Created a special error class for handling graecful rollbacks (not sure if Madeleine can take it yet)

libraries/instiki_errors.rb

@@ -0,0 +1,15 @@
+# Model methods that want to rollback transactions gracefully 
+# (i.e, returning the user back to the form from which the request was posted)
+# should raise Instiki::ValidationError.
+# 
+# E.g. if a model object does
+#   raise "Foo: '#{foo}' is not equal to Bar: '#{bar}'" if (foo != bar) 
+# 
+# then the operation is not committed; Rails returns the user to the page 
+# where s/he was entering foo and bar, and the error message will be displayed 
+# on the page
+
+module Instiki
+  class ValidationError < StandardError
+  end
+end