post-markdown build check for unexpected errors

[zkamvar]

One of the benefits of having error = TRUE by default is that we can see all of the errors at once instead of failing on the first error.

What I propose is that I implement a post-rendering check for errors (via {pegboard}) and report them as warnings to the author/CI unless they are generated from a chunk that has error = TRUE.

Originally posted by @zkamvar in https://github.com/carpentries/sandpaper/issues/305#issuecomment-1165739952