Error handling (and some other stuff)

Error handling now is done by [Gaffe](https://github.com/mirego/gaffe).

Using our own `ErrorsController` doesn't seem to work yet, so we disabled it for now. More infos here:
https://github.com/mirego/gaffe/issues/11

This PR also includes some other stuff that was originally implemented in the Synaesthesia project and then copy&pasted back to here (not the best workflow, but has to be enough for now).