`gitbook serve` crashes on error

[emilbayes]

Looking at Khan/KaTeX, there's an option throwOnError that could be set to false here. Otherwise the body of the process function should probably be wrapped in a try { … } catch (ex) { … }, if it's possible to tell Gitbook that an error occured

[SamyPesse]

This is wanted, when there is an error, we want gitbook to stop and output the error to the user.