v4 'cachePath' with absolute path

[nhitchins]

Expected behaviour

highcharts.cachePath or HIGHCHARTS_CACHE_PATH env var should allow an absolute path such as '/tmp/.cache'

Actual behaviour

Previously (v3) if the manifest.json file existed in the 'node_modules/highcharts-export-server/.cache' directory but read-only it would log an error and initPool would continue to start. Now initExport throws an error.

Reproduction steps

Set the 'node_modules/highcharts-export-server/.cache/manifest.json' file as read-only and initExport will throw an error.
Ideally just allow cachePath to specify an absolute path and if the manifest.json file exists (and un-changed) then skip writing new manifest

[DanielRuf]

All hardcoded paths should be configurable (with default values as fallbacks) in my opinion.

[jszuminski]

@nhitchins @DanielRuf feel free to test out #576 and let me know if absolute path for the Highcharts cache works as expected now.

[nhitchins]

@jszuminski Working now with absolute path - confirmed!

[jszuminski]

Great, thanks for the info! I will close this issue once we merge the PR.