OAuth Error gives user feedback

[CptPie]

This fixes #118.

I created a new struct

type callbackError struct {
    user         int
    message string
}

in the web package and added it to the webServer struct.

This new struct is used to "transmit" errors from the oauth callbacks to the frontend (a simple return would not work since the callback is executed by the routing and has no place to return the error to).

For further implementation details see the commit messages.