meta_analysis_BB missing exit code

When meta_analysis_BB fails with message similar to

```
Error: XML content does not seem to be XML: '3c3c277034c41f0547044//C141/physiboss_results/epithelial_cell_2_personalized_physiboss_run_1/final.xml'
Execution halted
```
 exit code is however 0 which should be fixed, so that it's set to non-zero if the BB fails