Summary:
This commit causes a desktop notification to be shown if we generate
EIO or ETIMEDOUT responses via fuse; the prompt is intended to make it obvious
to the user that they need to connect to the VPN.
The commit by itself doesn't show a notification, it allows configuring a
command that can be run to do something to show a notification.
The test plan includes one such configuration for our corp environment.
* It doesn't trigger for thrift-originated downloads (eg: prefetch), only for
VFS operations through FUSE.
* Ideally we'd know exactly when we have a network related error in the store
code and use that to trigger the notification. However, we have a rather
convoluted set of importers and fallbacks today, one of which is interpreting
a generic response returned from a pipe, so it is not especially clear
exactly where we should locate the logic
Reviewed By: chadaustin
Differential Revision: D17513364
fbshipit-source-id: 45134f3672679cb5580cb0c1bc12a0d6e38525ca