renderer: Helpful panic for unknown NodeKind

[abhinav]

(Follow up to #107.)

When rendering a Node, Renderer looks up the NodeRenderer for that
node's kind in a slice. That will panic with an "index out of range"
error if a node renderer for that node kind has not been registered.

panic: runtime error: index out of range [1] with length 1

This changes Renderer to panic with a more helpful error message if the
node kind is unrecognized.

panic: unrecognized node kind Document cannot be rendered:
register a renderer for this node kind first