warn & continue if Arduino serial conn open fails

Many users have run into the "cannot open /dev/" error message that so far occurs when the `duino` serial connection cannot be established.
That message isn't quite self-explanatory, and it's not obvious why an Arduino should be strictly required in the first place.

Now:
* a meaningful warning is displayed
* Heimcontrol does not quit immediately (though it's possible that it'll crash down the road if something expects the Arduino Board to be present; this could be improved further)

In other words, you can finally run Heimcontrol without an Arduino.

~~Depends on ni-c/duino#4.~~ Actually doesn't, uses my fork instead. This can be changed if the PR is accepted.
Fixes #64 & many similar.