Normal app termination causes Xpra to exit with error

[chowington]

Describe the bug
When a running app is intentionally ended, in some conditions it causes Xpra to end with an "X11 fatal IO error". This isn't ideal as it bubbles up through the container and ultimately the Kubernetes Job is shown as "Failed" rather than "Completed".

To Reproduce
Steps to reproduce the behavior:

1. Spin up an app locally
2. Connect to its Xpra window
3. Close the inner PyQt window by clicking its X button at the top right
4. Notice in the terminal that Xpra fails with "X11 fatal IO error"

Expected behavior
When the app is ended under normal conditions, all relevant processes should have an exit code of 0.