renderer: Don't panic for unknown NodeKind

[abhinav]

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.

This change verifies that a node renderer is available for that node
kind before attempting to use it, and instead of panicking, returns an
informative error message.

[yuin]

This kind of PR has been submitted before.

Works as intended. This is coding problem so it should panic.

[abhinav]

I can see the argument for this being a panic. The error message can be hard to decipher, though.

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

It definitely stumped me for a minute, which is what lead to this PR. The issue on my end was installing an extension on the parser but not on the renderer, which was instantiated separately.

I can update the PR to throw a panic with a more helpful error message if that's preferable.