Right now, the number of concurrent resharding operations allowed is capped (SERVER-136621), and when the max is reached, we return the error code ConflictingOperationInProgress which isn't reflective of the actual error (it is a relic of invoking a second resharding operation at all while one is ongoing). We should change it to something like ReshardingConcurrencyLimitReached or MaximumReshardingOperationsExceeded to be more reflective.

There is also a test plan for the multiple reshardings SPM-4934, but this ticket should also be sure to update the errors tolerated by the tests conditionally when the feature flag is on. it should grow the sets, not eliminate the old codes (probably).