Inconsistent Iteration Step Function gives nonesense error message

```
ExecutionEnvironment env = ExecutionEnvironment.getExecutionEnvironment();

DataSet<String> input = env.readTextFile(IN_FILE).name("source1");

IterativeDataSet<String> iteration1 = input.iterate(100);
IterativeDataSet<String> iteration2 = input.iterate(20);

iteration1.closeWith(iteration2.map(new IdentityMapper<String>())).print();
```
The code if wrong (closing iteration 1 with the step function of iteration 2), but it should not give the unsupported operation exception.

---------------- Imported from GitHub ----------------
Url: https://github.com/stratosphere/stratosphere/issues/893
Created by: StephanEwen
Labels:
Created at: Mon Jun 02 03:32:18 CEST 2014
State: open