Reimplement pageErrorHandler & menuErrorHandler option

[TurtlBbx]

Make possible to override the default error handling for page fetching and menu fetching in 2.x branch. Also document in readme how to to do that.

In 1.x branch it was documented like this:

• pageErrorHandler: The default page error handler can be overridden. Example:

  pageErrorHandler (error, commit, context, options) {
  context.error({
  statusCode: error.response.status,
  message: error.message
  })
  }

• menuErrorHandler: The default menu error handler can be overridden. Example:

  menuErrorHandler (error, commit, context, options) {
  commit('addMessage', {
  type: 'error',
  message: `Custom menu error: ${error.message}.`
  })
  }