Union of `isdisjoint` intervals

Right now, this is the error message we get when trying to compute the union of incompatible intervals:
```julia
julia> OpenInterval(1, 2) ∪ OpenInterval(2, 3)
ERROR: ArgumentError: Cannot construct union of disjoint sets.
```
It is a little bit confusing since it is possible to construct the union of disjoint intervals as long as they share an endpoint:
```julia
julia> isdisjoint(OpenInterval(1, 2), OpenInterval(2, 3))
true
julia> OpenInterval(1, 2) ∪ ClosedInterval(2, 3)
1 .. 3 (open-closed)
```
The confusion arises from such a pair of intervals being disjoint but connected sets. Maybe the error message should be changed to reflect that. Also, right now, in order to check if two intervals are disconnected, you have to do something like
```julia
isdisjoint(A, B) && !=(endpoints(A ∩ B)...)
```
which is a little bit inefficient since it computes `A ∩ B` twice or go full-on with
```julia
AB = A ∩ B
isempty(AB) && !=(endpoints(AB)...)
```
which is kind of lengthy. Maybe it would make sense to have a `isconnected` method?

All in all fairly minor issue, but it got me a little bit confused as a first-time user.