libstore/ssh: Improve error message on failing `execvp`

If the `throw` is reached, this means that execvp into `ssh` wasn’t
successful. We can hint at a usual problem, which is a missing `ssh`
executable.

Test with:

```
env PATH= ./result/bin/nix-copy-closure --builders '' unusedhost
```

and the bash version with

```
env PATH= ./result/bin/nix-copy-closure --builders '' localhost
```

(cherry picked from commit 38b29fb72ca4a07afbec1fd5067f59ca7d7f0fab)

0 files changed, 0 insertions, 0 deletions