Currently, a failing assert_label event prints out this:

INFO: 07/31/2007 02:55:48 PM Playing: AssertLabelEvent: output: abcabcdefFAIL
WARNING: 07/31/2007 02:55:48 PM Exception: java.lang.reflect.InvocationTargetException
com.thoughtworks.frankenstein.events.EventExecutionStrategy$1.execute(EventExec utionStrategy.java:25)
com.thoughtworks.frankenstein.events.AbstractFrankensteinEvent.play(AbstractFra nkensteinEvent.java:85)
com.thoughtworks.frankenstein.recorders.DefaultScriptContext.playAndReportTestS tatus(DefaultScriptContext.java:120)
com.thoughtworks.frankenstein.recorders.DefaultScriptContext.play(DefaultScript Context.java:81)
com.thoughtworks.frankenstein.recorders.DefaultScriptContext.playEvents(Default ScriptContext.java:100)
com.thoughtworks.frankenstein.recorders.DefaultScriptContext.play(DefaultScript Context.java:71)
com.thoughtworks.frankenstein.recorders.DefaultRecorder.run(DefaultRecorder.jav a:100)
java.lang.Thread.run(Unknown Source)

We need a better (simpler) error message, something like:
Playing: AssertLabelEvent
Failed: Unable to find label named "foo"