Custom emulators do not report failure to spawn

Set up an invalid customEmulator:

```
"dart.flutterCustomEmulators": [
	{
		"id": "danny_foo",
		"name": "Danny Foo",
		"executable": "~/Dev/foo",
		"args": [],
	}
],
```

The emulator fails to launch and writes to the log, but doesn't show anything on screen besides "Waiting for Danny Foo to connect".