Crash when running odoc_driver with an unexisting package name

Running the following command results in a crash:

```
$ odoc_driver foo
Deciding which packages to build...
odoc_driver: internal error, uncaught exception:
             Failure("Error: run ['opam' 'list' '--recursive' '-i' '--columns' 'package' '--color' 'never'\n     '-s' '--or' '--required-by' 'foo']: exited with 5")         
```