fix(vpn): handle non-file url on openvpn import

So openvpn import called to_file_path().unwrap(), which panics when the
file picker hands back a non-file:// url (sandboxed portal, etc). the
wireguard branch right above already handles this gracefully, so do the
same: return a Message::Error with a new OpenVpnConfigPath kind instead
of crashing.

Fixes #1896

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Signed-off-by: James A DellaMorte <dellamorte.james@comcast.net>
Co-authored-by: James A DellaMorte <dellamorte.james@comcast.net>

i18n/en/cosmic_settings.ftl

@@ -198,6 +198,8 @@ vpn-error = VPN Error
     .connect = Failed to connect to VPN
     .connection-editor = Connection editor failed
     .connection-settings = Failed to get settings for active connections
+    .openvpn-config-path = Invalid file path for OpenVPN config
+    .openvpn-config-path-desc = Chosen file must be on a local file system.
     .updating-state = Failed to update network manager state
     .wireguard-config-path = Invalid file path for WireGuard config
     .wireguard-config-path-desc = Chosen file must be on a local file system.