Add a server mode that renders errors

[Nadrieril]

Right now if there is an error while running neuron, the error is displayed on the terminal and the update doesn't show on the served pages. I would like an option that makes neuron display an error message on the served page instead. That way I can leave neuron running on a server and if a zettel fails to parse I will see it when I try to access the corresponding webpage.

[srid]

I've thought about this feature a few times, actually (albeit, to be able to handle more than parse errors). It'd certainly be nice to have.

[srid]

Looks like #201 will obviate this.

[Nadrieril]

Yep, this has now been implemented, thanks!