datastore: return error instead of unreachable

very unexpected and unreachable is probably fine here, but it's not
really winning us anything, so avoid the panic-potential and just
bail out.

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>

pbs-datastore/src/catalog.rs

@@ -518,7 +518,7 @@ impl<R: Read + Seek> CatalogReader<R> {
                 attr: DirEntryAttribute::Directory { start },
                 ..
             } => self.dump_dir(std::path::Path::new("./"), start),
-            _ => unreachable!(),
+            _ => bail!("unexpected root entry type, not a directory!"),
         }
     }