elarm functions could return whether the alarm already existed

We had an idea with Richard that elarm:raise and elarm:clear could return whether they raised/cleared an alarm or did nothing because the alarm was already existing/non-existing.

This way we could write code like this, which logs the problems only once:

```
case elarm:raise(...) of
    ok ->
        lager:error(...);
    existing ->
        ok
end.
```

@nygge, what do you think?
