Add report header to failure diagnostics

Add a one line header of JSON info about the failure in failure
diagnostics reports.

This can then be automatically collected to calculate statistics about
particular failures, for example how often they happen when wifi is not
connected, not validated etc.

Test: atest
Change-Id: Ic4c99c53fc2686aaf2f1651522aa2f0d82d0cb86
2 files changed