Misconfigured tile layers (e.g. under <datadir>/gwc-layers/) prevent server startup.
This could be due to a parsing error, or because the tile layer info points to a non existent layer or layergroup.
Expected behavior is to:
ignore layers that can't be parsed, and log a warning
disable layers that can be parsed but point to a non-existent layer/group; showing a message in the UI (GeoServerTileLayer has a configErrorMessage to that end)