Having main-is incorrectly populated causes repeated crashes that locks up your buffers

**How to reproduce**
populate main-is with a file that cannot be found, save and attempt to edit any of the .hs files in your project.

**What happens**
An error message occurs, stating that the haskell process has died and it prompts if you would like to restart it. Upon restarting it immediately crashes again

**What should (ideally) happen**
I don't think it would be much trouble to put a good error message in the mini buffer stating the main file could not be found, but this did leave me scratching my head for a few minutes wondering what was going on.

Maybe this is intended behaviour, it just seems like a bad user experience to repeatedly crash, maybe a better fix would be to keep a log of the last 3 crash logs from cabal and if it crashes the same way 3 times to stop re-attempting or something?

Reproduce the failure by setting main-is to a missing file, saving, and editing a .hs file in the project. Start at the Haskell process restart and crash-report path, including the minibuffer message and Cabal crash logs; done means a missing main file is reported clearly without repeated crashes or restart prompts.

Written by the indexing model from the issue text.