Feature request: Crash at startup when local storage is not writable

[iTrooz]

I am using a local storage to store vsix files (ovsx.storage.local.directory)

When I initially set this configuration option, the app was VERY buggy (e.g. extensions showing as "deactivated", and "Extension not found" errors when trying to access a deployed extension), and I found out later this was because the directory I had set for storage was not writable.

It would be great if the application crashed at startup in this case, rather than having weird behaviours at runtime (none of which showed an error in the webui)