ReBench commandline exit code should not be non-zero when there's nothing to do

If we find that all data is already collected, it's a bit odd that ReBench indicates an error with the return code.

To reproduce, do something like this:

```bash
rebench -R rebench.conf 's:*:List'
rebench -R rebench.conf 's:*:List'
```

The first run should succeed on for instance a SOM, and exits with 0.
The second time exits with exit code 1, even so, it has all data already.