show alert notification if on Read Only directory

[getneil]

Problem:

Sentry Error:

Cannot update while running on a read-only volume. The application is on a read-only volume. Please move the application and try again. If you're on macOS Sierra or later, you'll need to move the application out of the Downloads directory. See https:...

This is caused when user installing the GUI intentionally puts the GUI/APP in the downloads folder instead of the /Applications.
When users do this, they remove the app's write permissions thus disabling it from doing anything.

[owenjames]

use modal to show that it's not possible to work in the downloads folder (or read only)