We should validate the shell path exists and cache the result per window. Need to make sure it also works with symlinks.

There is this code that is meant to do this sort of thing right now but I think it's Windows only and also still just says "exit code 2":

https://github.com/Microsoft/vscode/blob/50c9a782ad2cf3b1b662966455187f8a9ec41bac/src/vs/workbench/contrib/terminal/node/terminalProcess.ts#L71-L77