change blocktree*::storage_size() to return Option<u64> to handle live fs changes (#7401)

ledger/src/blocktree.rs

@@ -1493,7 +1493,7 @@ impl Blocktree {
         self.last_root()
     }
 
-    pub fn storage_size(&self) -> u64 {
+    pub fn storage_size(&self) -> Option<u64> {
         self.db.storage_size()
     }
 }

ledger/src/blocktree_db.rs

@@ -580,8 +580,8 @@ impl Database {
         self.backend.write(batch.write_batch)
     }
 
-    pub fn storage_size(&self) -> u64 {
-        get_size(&self.path).expect("failure while reading ledger directory size")
+    pub fn storage_size(&self) -> Option<u64> {
+        get_size(&self.path).ok()
     }
 }