wrapCallback doesn't work with single-argument-callbacks

WrapCallback provides a callback function which expects two arguments:
1. An error/null.
2. The actual value.

This doesn't work with functions like fs.exists or http.get because they call the function only with the result. [See the code in question](https://github.com/caolan/highland/blob/d7fbb1d03c23a8817fb121e34bff40585b2ff964/lib/index.js#L3985-L3993).

This behavior breaks the example of flatFilter():

``` javaScript
var checkExists = _.wrapCallback(fs.exists);
filenames.flatFilter(checkExists)
```

This code will always throw 'Uncaught, unspecified "error" event.'

Start with lib/index.js lines 3985-3993 and trace how wrapCallback handles callback arguments and errors. Reproduce the flatFilter example with fs.exists or http.get, then confirm that single-result callbacks work without breaking the documented error/value callback form or emitting an unspecified error.

Written by the indexing model from the issue text.