Updated "getFile" to send a 404 for ENOENT errors

getFile sends a 500 error response when the requested file cannot be
found, but the status code probably should be 404. This change sets
the status of the error to 404 when the error code is "ENOENT".

lib/storage-service.js

@@ -210,6 +210,10 @@ StorageService.prototype.getFiles = function(container, options, cb) {
  */
 StorageService.prototype.getFile = function(container, file, cb) {
   return this.client.getFile(container, file, function(err, f) {
+    if (err && err.code === 'ENOENT') {
+      err.statusCode = err.status = 404;
+      return cb(err);
+    }
     return cb(err, map(f));
   });
 };