Return more correct value for get_pgroup

Down the line, the return value of get_pgroup is passed to `strerror`. As such, it should probably be a proper errno value rather than -1. In cases where someone tries to reparent a non-existant pid, this changes the error message from:

```
[-] Unable to get pgid (does process 26197 exist?)
Unable to attach to pid 26197: Unknown error -1
```

to

```
[-] Unable to get pgid for pid 26197
Unable to attach to pid 26197: No such process
```

which seems more correct.