Improvements to the matrix cache (#1340)

* Improvements to the matrix cache

- Add a timelimit for the matrix in the cache.
Once the timelimit is reached, the matrix can be removed.

- If a DatasetAccessError occurs, then remove the dataset
from the matrix cache.

Fixes #1322

server/data_cxg/cxg_adaptor.py

@@ -250,6 +250,8 @@ class CxgAdaptor(DataAdaptor):
         with ServerTiming.time(f"layout.lsuri"):
             pemb = self.get_path("emb")
             embeddings = [os.path.basename(p) for (p, t) in self.lsuri(pemb) if t == "array"]
+        if len(embeddings) == 0:
+            raise DatasetAccessError("cxg matrix missing embeddings")
         return embeddings
 
     @staticmethod