Track both device and active connections at the same time and decide
whether activation finished (either successfully or not) in a single
place, check_activated().
This makes the already too complex code path a bit more straightforward
and also makes it possible to be more reasonable about the diagnostics.
Now that the active connection signals the reason, we include it; but if
the failure is due to the device disconnection while we're activating,
include a device reason instead, since it's often more useful. Like:
Before:
Error: Connection activation failed.
Without considering the device:
Error: Connection activation failed: the base network connection was interrupted
After:
Error: Connection activation failed: The Wi-Fi network could not be found
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