fixed issue #59

added verification for duplicated traditional path
added new unit test for verification
modified existing unit test which results error with verification
Instead of this, you can set ``error`` variable inside the Jx9 script. An exception will be raise automatically after the execution of the script. (Take a look at ``execute_vm`` method.)

This way it raises ``DatabaseScriptLogicalError``, which should be raises if script was OK but there was a logical error in the client's request. Like duplicated traditional path.