backups: remove entries from database that don't exist in storage

fixes #772

src/storage/noop.js

@@ -1,22 +1,23 @@
 'use strict';
 
 exports = module.exports = {
-    getBackupPath: getBackupPath,
-    checkPreconditions: checkPreconditions,
+    getBackupPath,
+    checkPreconditions,
 
-    upload: upload,
-    download: download,
-    downloadDir: downloadDir,
-    copy: copy,
+    upload,
+    exists,
+    download,
+    downloadDir,
+    copy,
 
-    listDir: listDir,
+    listDir,
 
-    remove: remove,
-    removeDir: removeDir,
+    remove,
+    removeDir,
 
-    testConfig: testConfig,
-    removePrivateFields: removePrivateFields,
-    injectPrivateFields: injectPrivateFields
+    testConfig,
+    removePrivateFields,
+    injectPrivateFields
 };
 
 var assert = require('assert'),
@@ -49,6 +50,16 @@ function upload(apiConfig, backupFilePath, sourceStream, callback) {
     callback(null);
 }
 
+function exists(apiConfig, backupFilePath, callback) {
+    assert.strictEqual(typeof apiConfig, 'object');
+    assert.strictEqual(typeof backupFilePath, 'string');
+    assert.strictEqual(typeof callback, 'function');
+
+    debug('exists: %s', backupFilePath);
+
+    callback(null, false);
+}
+
 function download(apiConfig, backupFilePath, callback) {
     assert.strictEqual(typeof apiConfig, 'object');
     assert.strictEqual(typeof backupFilePath, 'string');