Fix: Crash when the specifed scalar was not present on one input

When one of the inputs was missing the specified scalar, the filter crashed for 2 possible reasons.

• If it was on the first input, GetName() method was called on nullptr.
• In other cases, the imagesOperations filter returned an invalid array it was not handled by ImageBasedClustering
• Changes the VTK_ERROR by return 2 as is was considered an OK value by the vtk pipeline