Connecting to binder can appear to fail with a CORS error

[stevejpurves]

Describe the bug

context
When i use the demo apps and try to connect to binder for the first time, the connection fails, a CORS message is displayed in the javascript console.

However, when following the binder link from the console and opening the event stream, the server is launching and data is coming in from the event stream.

It seems like potentially there is an early http response code error, returned before binder proceeds with the connection. Perhaps previously thebe was not sensitive to this. It seems like this might only occur when an image is not available on binder, as when retesting after at least one binder session has launched the connection proceeds smoothly.

expectation
I expected the connection to proceed smoothly without any errors

bug
But instead the connection appears to fail

Reproduce the bug

build either of the thebe demos and connect ot binder with details for a repo that has either been culled or not used before

List your environment

No response