toPromise() does not propagate errors properly

There are certain situations where the toPromise method will resolve successfully with a highland internal error being thrown.

to recreate...
```javascript
_([1])
.map(x => _.nil)
.toArray(arr => console.log);
```
Will error `Error: Cannot call next after nil`

however....
```javascript
_([1])
.map(x => _.nil)
.collect()
.toPromise(Promise)
.then(console.log);
```
Will just log an empty array to the console.

Start by tracing the toPromise and collect entry points using the reproduction in the issue, then compare their error handling with the toArray behavior. Done means the Promise rejects with the Highland internal error instead of resolving with an empty array.

Written by the indexing model from the issue text.