ClientError will default to 400 HTTP error code.
ServerError will default to 500 HTTP error code.
Following the previous commit, these for now are only being used in
albumsController. More will soon follow.
Additionally fixed existing album names can sometimes be re-used when
editing an album.
This will be used for errors that are to be delivered to users, AND not
to be logged into the server (as in it stacktraces and all).
This will eventually remove the need to throw string literals.
In this commit, this has only been implemented on albumsController.js,
but more will soon to come.
