Check if scheme can be opened before opening

This commit adds a check to the helper that will retrieve the path to the app that responds to the given scheme. Failure to retrieve this path means the app does not exist on the system and the scheme shouldn't be called.

It's a fix for a relatively rare case when the container app has been moved to the Trash, but the helper is still in launchd. The OS will throw errors stating the scheme can't be opened.