start_dashboard hangs if there aren't two available ports in the provided range

[harmenwassenaar]

The main process simply keeps waiting for DASHBOARD_STARTED_EVENT to be set, which never happens when the "all ports used" exception occurs in the child process.

[sybrenjansen]

Ah yes, I see. Seems you can easily fix it by altering the start_dashboard function to something like this:

while not DASHBOARD_STARTED_EVENT.wait(timeout=0.01):
    if process.join(timeout=0.01):
        raise ....

If the process joins, then apparantly an exception occurred.

Perhaps storing the exception of the _run function and raising that is better, but it requires more work.