chore: clear session on launch if vpn unconfigured

[ethanndickson]

Logging into Coder Desktop configures the system VPN with the session token & server URL.

However, uninstalling the app does not remove clear the UserDefault settings (namely hasSession), nor does it clear the keychain item (containing the session token). This means a fresh install of the app (that involves deleting the network extension from settings) starts in a broken state:
The VPN looks like it can be used, as typically the NE is configured once logged in, but the deleting of the network extension cleared the configuration. Right now we show an error that suggests logging out and back in.

This PR makes it not possible to enter this broken state. If a NE configuration cannot be loaded on start up of the app, the session is cleared, and the user will see they need to log in again.

As a drive-by fix, this PR also adds descriptions to all errors, as some varieties of errors would not display a description on the UI.

[ethanndickson]

• chore: clear session on launch if vpn unconfigured #76 👈 (View in Graphite)
• main

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