tryIO for cases when the action *both* uses Either *and* exceptions

I ran into an issue today where a library (`HTTP`) was using `Either` but could _also_ produce an exception. I ended up writing this, though something that ends up in `EItherT` might also be reasonable:

```
tryIO' :: (IOError -> e) -> IO (Either e b) -> IO (Either e b)
tryIO' mapErr io = eitherT (return . Left . mapErr) return (tryIO io)
```

Start by reviewing the existing tryIO and eitherT entry points mentioned in the issue, then determine where a tryIO' helper belongs. Done means exceptions from the IO action are mapped with the supplied function into Left, while existing Either values remain unchanged.

Written by the indexing model from the issue text.