Fix typo causing error when incorrect param entered.

When entering an incorrect N value in the HPR model tab (i.e. not a power of two), the code fails because the exception has been declared with a typo:

```
Exception in Tkinter callback
Traceback (most recent call last):
  File "/usr/lib/python2.7/lib-tk/Tkinter.py", line 1489, in __call__
    return self.func(*args)
  File "/home/sebastien/dev/sms-tools/software/models_interface/hprModel_GUI_frame.py", line 204, in compute_model
    tkMessageBox.showerror("Input values error", errorMessage)
NameError: global name 'errorMessage' is not defined
```