Custom internal server error messages

[mrkishi]

Sapper will currently bail with <pre>Internal server error</pre> when errors are thrown while rendering error pages.

This was done to prevent the node process from hanging in an infinite loop, but we should probably look into handling it more gracefully.

At the very least, we could support customizing the error with a static template-internal-error.html akin to template.html when nothing else can be done.

Thoughts?