Do not quit Jamulus if no valid audio device was found

**Has this feature been discussed and generally agreed?**
No. But a I assume many people are aware of it

**Describe the solution you'd like**

If you select devices with a wrong sample rate in e.g. ASIO4ALL or misconfigure devices, Jamulus currently handles this as fatal error (See soundbase.cpp SetDev). Users of ASIO4ALL might select a device which doesn't support 48kHz. This results in a warning and a fatal error. Jamulus will then ask you to close the app, even if the driver settings are open. Also you can't even open Jamulus if something is wrong with your audio devices since it quits with an error message. #305 fixed some parts of this issue, but in my opinion the current handling is not user friendly. I'd expect:

1. A clear error message that the current device is not supported in this state which doesn't disappear unless you fixed the issue.
2. A clear hint how to fix this issue (but sometimes that's not easy)
3. Jamulus not being quit. This confuses a lot of users. In my opinion it's not an error which should result in Jamulus being quit. Maybe we could even allow the user to change basic settings or connect to a server with chat only?

See: https://github.com/jamulussoftware/jamulus/issues/305#issuecomment-776106394

Start with soundbase.cpp and SetDev, then review issue #305 and the closed pull request #1378 for prior context. Done means an unsupported or misconfigured audio device produces a persistent, actionable message without forcing Jamulus to quit.

Written by the indexing model from the issue text.