Slots should not throw

I just found out that *Throwing an exception from a slot invoked by Qt's signal-slot connection mechanism is considered undefined behaviour, unless it is handled within the slot* ([here](https://doc.qt.io/qt-5/exceptionsafety.html#signals-and-slots)).

We should probably declare all of [these](https://github.com/CanonicalLtd/multipass/blob/4bd6a5885e2ed851cb8971b45dcd686fb4bca967/src/daemon/daemon.h#L77) noexcept and perhaps add a `catch(...)` (only std::exception is caught ATM).

Other *callables* used as slots could also do with a review and be similarly safe-guarded.

Start with the slots listed in src/daemon/daemon.h around line 77 and inspect the existing exception handling, including the current std::exception catch. Review the other callable slots mentioned in the issue; done means exceptions from these signal-slot paths are handled safely rather than escaping the slot.

Written by the indexing model from the issue text.