[SheepShaver] Prevent launch if non-existent volume is listed in Settings

[enderandpeter]

Sheepshaver should prevent starting the VM if one of the paths listed in the Volumes tab cannot be found. Sometimes, after reshuffling hard drives on my Windows machine, paths will be listed for volumes I've added that are now on a different drive, and it is often confusing when Sheepshaver does not provide any feedback after clicking Start and the Settings window goes away. Might you consider a feature like making a path name red, perhaps, if it can't be found when the launcher appears, or disabling the Start button if there is a non-existant Volume path?

I'm using the 06-05-2013 build of SheepShaver for Windows 10, but I first noticed this a couple years ago. This kind of emulation is a dream come true, by the way, so thank you very much for keeping up with this project.

In case it helps, I've shared the error messages from Event Viewer.