pdal_wrench exits with `0` even when there was an error

pdal_wrench exits with `0` even when there was an error.

When there is no `command` passed, exit code is properly `1`
When a command is used, exit code is `0` regardless of success state:
```bash
./pdal_wrench &> /dev/null; echo $?
1
./pdal_wrench asdf &> /dev/null; echo $?
1
./pdal_wrench merge &> /dev/null; echo $?
0
./pdal_wrench merge asdf &> /dev/null; echo $?
0
```
