Bad error handling: unhelpful "unexpected" error message when parsing set of strings

A parser like

```
parser :: GenParser Char st String
parser = choice $ fmap (try . string) ["head", "tail", "tales"]
```

with malformed input "ta " will produce an error message like `unexpected "t"` and point to the first position of the input. Better output would be `unexpected " "` (and possibly a list of expected outputs, `["tails", "tales"]` in this case).

Please see this much more detailed [StackOverflow answer/analysis](http://stackoverflow.com/a/34358623/1260229) written by _CR Drost_ (starting with the second headline).