If we cannot find a BMC proxy for the hosts subnet, report an error

Instead of using a random proxy that might not work.

Before
```
500 Internal Server Error
```

Expected (with this change)
```
Failure: ERF51-1518: Unable to find a proxy with BMC feature
```

See also https://github.com/theforeman/foreman/commit/40df7dfbf#commitcomment-4543636

I'll open a Redmine for this soon!