hh_client rapid restart loop when `.hhconfig` parsing calls `failwith`

I was hoping to use `forward_compatibility_level` as a nice way of alerting people when their local hhvm installation is out of date, instead of `hh_client` returning a lot of errors to them because the code depends on a newer version. However this isn't practical in 3.29 or 3.28 due to this issue.

### HHVM Version

```
$ hh_client --version
hh-d968c089ca41c9afc68c35c8fabcb4154f12d2bc-3.29.0 Mon Oct 22 09:53:31 2018
```

### Operating System and Version

MacOS High Sierra

### Standalone code, or other way to reproduce the problem

in `.hhconfig` with an empty repo:

```
forward_compatibility_level = 3.30
```

... or ... (edit)

```
enable_tc_migration_flags=foo
```

Then run `hh_client`

### Actual result

hh_client rapidly loops and restarts infinitely, it's impossible to get it to stop without killing the terminal tab or restarting

### Expected result

~~A useful error is printed stating that your hhvm version is below the required version.~~

Edit: failwith message is printed