all: Propagate errors from NamespacedKV (#6203)

As foretold by the prophecy, "once the database refactor is merged, then
shall appear a request to propagate errors from the store known
throughout the land as the NamedspacedKV, and it shall be good".

lib/fs/mtimefs_test.go

@@ -241,7 +241,7 @@ func (s mapStore) PutBytes(key string, data []byte) error {
 	return nil
 }
 
-func (s mapStore) Bytes(key string) (data []byte, ok bool) {
+func (s mapStore) Bytes(key string) (data []byte, ok bool, err error) {
 	data, ok = s[key]
 	return
 }