Make `osrm-cli` more robust against missing arguments

As reported on IRC by user winwaed_, `osrm-cli` has the default behavior of horribly crashing.

```
daniel@x1c /t/o/build> ./osrm-cli
[info] starting up engines, v4.7.1-1-g40443d1
[warn] caught exception: Start canary of block corrupted.
```

Better way of handling missing arguments would be to print the `--help` output.
