torture: Make kvm-recheck.sh report .config errors

Currently, kvm-recheck.sh will print out any .config errors with messages
of the form:

:CONFIG_TASKS_TRACE_RCU=y: improperly set

However, if these are the only errors, the resulting exit code will
declare the run successful.  This commit therefore causes kvm-recheck.sh
to record .config errors in the results directory in a file named
ConfigFragment.diags and also returns a non-zero error code in that case.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>