stack-mode won't work

Two issues, possibly related.

First, there is a message:

```
File mode specification error: (void-function haskell-cabal-find-file)
```

This can be fixed (I guess), by adding

```
(require 'haskell-cabal)
```

to `stack-mode.el`, but then I got:

```
Stack backend isn't ready. Waiting (0 attempts) ...
Starting a Stack IDE backend process for this project: geval, stack directory: /home/filipg/ext/geval/
Set initial command.
Wrote /home/filipg/ext/geval/src/GEval.hs
error in process filter: cond: Command handler error: (wrong-type-argument stringp nil)

The command queue has been cleared.
error in process filter: Command handler error: (wrong-type-argument stringp nil)
```

(GEval is my piece of code.) The last line in `*stack:geval*` is:

```
<- {"tag":"ResponseGetSourceErrors","contents":[{"errorSpan":{"tag":"TextSpan","contents":""},"errorKind":"KindError","errorMsg":"can't find file: /home/filipg/ext/geval/GEval"}]}
```

Emacs: 24.5.1, haskell-mode: 13.14.2, flycheck: 0.24, stack-ide: d096461bb4d2e800f67a934000a31125e95bf0be