improve error message for run.native tests.jit.only

[kylegoetz]

The command right now (for my work implementing UDP for the JIT) currently spits out the following when testing

```ucm
    runtime-tests/selected> run.native testsruntime-tests/selected> run.native tests.jit.only
    ```
    
    
    🛑
    
    The transcript failed due to an error in the stanza above. The error is:
    
    
      native evaluation failed
    
    
  Run `unison --codebase /Users/kylegoetz/.cache/unisonlanguage/runtime-tests.unison` to do more work with it.

Not a terribly useful error.

[kylegoetz]

Though I'm not sure how it could be improved upon easily. The code that generates the error is

ExitSuccess ->
            decodeResult . deserializeNativeResponse
              =<< UnliftIO.takeMVar mv
          ExitFailure _ -> do
            UnliftIO.killThread tid
            pure . Left $ "native evaluation failed"

and ExitFailure wraps an INT only.