Improve "Lost network stack" error message

The "Lost network stack" message often gets reported as suspected
root cause of a crash, but it is actually just a side-effect of a crash
of the network_stack process, which often was just killed together with
the rest of the system.

Clarify the error message, so that it is clear that the root cause
should be visible earlier in logs.

Bug: 198681439
Change-Id: Ieb79ace34170f8bd9ea847175199590998278d57
Test: m