error message should be smarter

When the command specified as input is unambiguously a snap command/snap name, we shouldn't require explicitly setting --use-snap-run, i.e. with only libreoffice installed I see this:

```
$ etrace exec --clean-snap-user-data --reinstall-snap --discard-snap-ns --no-trace libreoffice
cannot use --discard-snap-ns without --use-snap-run
```
