NE: Change handling of bad domain names and Activate On Demand

NE: Change handling of bad domain names and Activate On Demand

The solution implemented in commit b8c331c causes the tunnel to
remain in 'Activating' state, without the ability to cancel that.

So, in this commit, instead of retrying DNS silently on
Activated-On-Demand tunnels, we fail the startTunnel() silently.

To summarize, if activate-on-demand is on:
- If started from the WireGuard app, show error using lastErrorFile
mechanism, suggesting a way to turn off Activate On Demand
- If not started from WireGuard app, don't call displayMessage()
(don't show error to user) and silently fail starting the tunnel

Signed-off-by: Roopesh Chander <roop@roopc.net>