In case the corresponding backend is not installed and the user tries to open a file produced on another system where this backend was available, Cantor doesn't handle this case very well. See "steps to reproduce".

Reproducible: Always

Steps to Reproduce:
1. Open a file for a backend that is not available on the system.
2. Cantor reports problems with initializing the modul and the dialog with "session is being initialized" never disappers. After hitting the cancel button in this dialog, there is still "computation is running" message in the status bar. The outputs in the opened file are not shown correctly.


Expected Results:  
Cantor should report the missing backend, show the saved file correctly and disallow any computations in the worksheet. The file should be opened in the read-only mode.