Handle error of browserify's bundle callback properly.

`err` to `onBundleComplete` was not properly handled and caused the following error.

```
Uncaught TypeError: First argument needs to be a number, array or string.
  at new Buffer (buffer.js:188:15)
  at onBundleComplete (/Users/s-kagawa/work/js/gulp-browserify/index.js:90:31)
  at Stream.<anonymous> (/Users/s-kagawa/work/js/gulp-browserify/index.js:108:21)
  at Stream.<anonymous> (/Users/s-kagawa/work/js/gulp-browserify/node_modules/browserify/index.js:232:22)
  at Stream.<anonymous> (/Users/s-kagawa/work/js/gulp-browserify/node_modules/browserify/index.js:232:22)
  at Stream.EventEmitter.emit (events.js:117:20)
  at Stream.EventEmitter.emit (events.js:117:20)
  at Stream.EventEmitter.emit (events.js:117:20)
  at /Users/s-kagawa/work/js/gulp-browserify/node_modules/browserify/node_modules/module-deps/index.js:91:36
  at /Users/s-kagawa/work/js/gulp-browserify/node_modules/browserify/index.js:570:20
  at /Users/s-kagawa/work/js/gulp-browserify/node_modules/browserify/node_modules/browser-resolve/index.js:183:24
  at /Users/s-kagawa/work/js/gulp-browserify/node_modules/browserify/node_modules/browser-resolve/node_modules/resolve/lib/async.js:36:22
  at load (/Users/s-kagawa/work/js/gulp-browserify/node_modules/browserify/node_modules/browser-resolve/node_modules/resolve/lib/async.js:54:43)
  at /Users/s-kagawa/work/js/gulp-browserify/node_modules/browserify/node_modules/browser-resolve/node_modules/resolve/lib/async.js:60:22
  at /Users/s-kagawa/work/js/gulp-browserify/node_modules/browserify/node_modules/browser-resolve/node_modules/resolve/lib/async.js:16:47
  at Object.oncomplete (fs.js:107:15)
```

Browserify emits `error` event when it handles stream entries. However, it gives an error to `bundle()`'s callback function when it handles non-stream entries.