ipn/desktop: skip the extension when sessions can't be enumerated

Registering the session state callback enumerates the existing desktop
sessions, which fails for a tailscaled run by an unprivileged user
(WTSEnumerateSessions returns "No more data is available"). That was
reported as "init failed", which reads like a bug. Such a tailscaled has
no other users' sessions to track, so report it as skipping the extension,
the same way an unavailable session manager already is.

Updates #2791

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
Change-Id: I7f9b1d3e5a7c9e1b3d5f7a9c1e3b5d7f9a1c3e5b
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