When performing an external action that fails due to an error in the environment, the error is not caught so it is not clear why it fails. This could be solved by catching the Exception that is thrown and printing the error message in the 2apl interpreter.
To be a little more precise: in the class apapl.Plans.ExternalAction the method execute(APLModule) will execute an external action by calling the method execute(APLFunction, Environment, APLModule) in the same class. This last method throws an ExternalActionFailedException (which is the exception that also can be thrown when creating a method in an environment), but this exception is not caught in the first method that we mentioned.