Silent failure when ghc-mod fails

Running GhcModCheck in the case where ghc-mod fails (upstream issue https://github.com/DanielG/ghc-mod/issues/915) leads to

```
ghc-mod check: No errors found
```

When really ghc-mod completely failed. I don't think this is good behaviour, perhaps showing something like

```
ghc-mod check: Unexpected failure
```

or similar would be more instructive.