This expands the reach of error propagation greatly throughout the
kernel. Sadly, it also exposes the fact that we're allocating (and
doing other fallible things) in constructors all over the place.
This patch doesn't attempt to address that of course. That's work for
our future selves.
This makes for nicer handling of errors compared to checking whether a
RefPtr is null. Additionally, this will give way to return different
types of errors in the future.
