du: better error handling of du

du can fail when files and directories go missing. luckily, when du fails,
it still provides the best effort output

src/storage/filesystem.js

@@ -40,10 +40,10 @@ const assert = require('assert'),
     safe = require('safetydance'),
     shell = require('../shell.js')('filesystem');
 
-// the du call in the function below requires root
 async function getAvailableSize(apiConfig) {
     assert.strictEqual(typeof apiConfig, 'object');
 
+    // note that df returns the disk size (as opposed to the apparent size)
     const [error, dfResult] = await safe(df.file(apiConfig.rootPath));
     if (error) throw new BoxError(BoxError.FS_ERROR, `Error when checking for disk space: ${error.message}`);