updater: better error message

src/df.js

@@ -2,13 +2,24 @@
 
 exports = module.exports = {
     disks,
-    file
+    file,
+    prettyBytes
 };
 
 const assert = require('assert'),
     BoxError = require('./boxerror.js'),
     safe = require('safetydance');
 
+// binary units (non SI) 1024 based
+function prettyBytes(bytes) {
+    assert.strictEqual(typeof bytes, 'number');
+
+    const i = Math.floor(Math.log(bytes) / Math.log(1024)),
+        sizes = ['B', 'KB', 'MB', 'GB', 'TB', 'PB', 'EB', 'ZB', 'YB'];
+
+    return (bytes / Math.pow(1024, i)).toFixed(2) * 1 + '' + sizes[i];
+}
+
 function parseLine(line) {
     const parts = line.split(/\s+/, 7); // this way the mountpoint can have spaces in it