core/alet works in unexpected way

Hi!

Seems like when one of `alet` args in not wrapped into monad it returns the value of the last of that arg.
Better show than explain:

```
err.core=> (alet [a (ei/right 1) b (inc a)] 42)
2
err.core=> (alet [a (ei/right 1) b (ei/right (inc a))] 42)
#
err.core=> (alet [a (ei/right 1) b (inc a) c (+ b 4)] 42)

IllegalArgumentException No context is set and it can not be automatically resolved from provided value cats.context/throw-illegal-argument (context.cljc:32)
err.core=> (alet [a (ei/right 1) b (inc a) c (+ a 4)] 42)
2
```

I think it should raise exception in all that cases.