Error message for non existing materials in the Encyclopedia

Due to our delayed indexing, some materials that are referenced in the repository might not exist in the encyclopedia. The Encyclopedia GUI needs to produce a meaningful error message, e.g. "Material not yet available."

For example, if you click ENCYCLOPEDIA here on the structure: https://nomad-lab.eu/prod/rae/beta/gui/entry/id/10k9kKRNTiWFsaK56Ft3og/xUjNvzui8NTYPBhdxshs5QukaRms/overview