Fixed exceptions thrown from the user's callback being silently swallowed

Example

```js
ghpages.publish('dist', function (err) {
  if (err) throw err;
})
```

Behavior in version 0.2.0: the exception is silently swallowed and the program finishes normally without any error or warning. The outcome is the same if the silent option is enabled.

Behavior with this PR: the exception is thrown and the program stops with the error message and stacktrace. If the silent option is enabled, the error message is *Unspecified error (run without silent option for detail)*, as expected.