ci-demand: an unwritable cache is only a stderr warning and resume advice is wrong #120

When the cache folder can't be written to, ci-demand only prints a warning on stderr. The --json output does not show it, and a stopped run still says "run again with the same --cache", which does not help because nothing was saved. On a big date range the run can then never finish.

Fix: report the unwritable cache in the JSON output, and do not suggest resuming when nothing was saved (or refuse to start a long run without a working cache). A failing test was written during the review.

From the quick-check review of #113: #113 (comment)

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