Better error when cgroup time isn't readable.
Better error when cgroup time isn't readable.

Previously, this would bail out with a confusing KeyError. Now at least
the raised exception mentions the actual problem, which would help
someone debug the problem.

Tested by replacing 'time' with a garbage string.

google/blkcgroup,akshaylal/blkcgroup,akshaylal/blkcgroup,google/blkcgroup