log: Use LogWarning for non-critical logs

As per doc/developer-notes#logging, LogWarning should be used for severe
problems that do not warrant shutting down the node

src/node/blockstorage.cpp

@@ -1263,7 +1263,7 @@ void ImportBlocks(ChainstateManager& chainman, std::span<const fs::path> import_
                 return;
             }
         } else {
-            LogPrintf("Warning: Could not open blocks file %s\n", fs::PathToString(path));
+            LogWarning("Could not open blocks file %s", fs::PathToString(path));
         }
     }