Feature
* At groupspec init, iterate over values_loader and check that they exist in the loader registry ([`04dfd7e`](https://github.com/Aarhus-Psychiatry-Research/timeseriesflattener/commit/04dfd7e7e038472cfd26f67c79a6b050cc13b15e))
Fix
* More explanation in error message ([`b784991`](https://github.com/Aarhus-Psychiatry-Research/timeseriesflattener/commit/b7849911c85ca6ac5bd165b7a48ccce1a768f70b))
* Bettee valueerror message formatting ([`7b3b994`](https://github.com/Aarhus-Psychiatry-Research/timeseriesflattener/commit/7b3b994cbe38df73a4149c4463b5f283ad297218))
* Better valueerror message ([`d92f798`](https://github.com/Aarhus-Psychiatry-Research/timeseriesflattener/commit/d92f7989af27a879fd090bed33ce5027e96e581b))
* Find invalid loaders ([`ba2d4c5`](https://github.com/Aarhus-Psychiatry-Research/timeseriesflattener/commit/ba2d4c540f097c33ca5c29a0b72a908ad6dc04e3))