config_map.erase() is only called in ExportHandler::finish_timespan().
When an export fails (throw) or is aborted, the export-handler's
config remains as is and the next export will run it again.
The export-handler is global, per session and
ExportHandler::add_export_config() only ever inserts or ignores insert.
This is in preparation to fix:
1) export to invalid path -> fail, error is thrown
2) correct path -> new config is inserted in the map
3) try to export again, first runs the not-completed export from (1)
-> constant errors.
506863bf57