storage: Make sure abort method exists before calling

[stephenplusplus]

While playing around today, I noticed an error can occur before an API request is made. It's only on an HTTP stream that we get the abort method (to close the connection), so we have to check that it's available to call first.

From lib/storage/file.js:

  requestStream
      .on('error', function(err) {
        throughStream.destroy(err);
      })
      .on('response', function(res) {
        throughStream.emit('response', res);
        util.handleResp(null, res, null, onResponse);
      })
      .on('complete', function(res) {
        util.handleResp(null, res, null, onComplete);
      })
      .pipe(throughStream)
      .on('error', function() {
++      if (requestStream.abort) {
          requestStream.abort();
++      }
        requestStream.destroy();
      });