Module mounts should cause an error for a bad source or includeFiles

This is valid, and causes no errors:

[[module.mounts]]
source = "node_modules/katex/dist"
target = "assets/paige/katex"
includeFiles = ["/katex.css"]

If you change source to node_modules/katex/distWRONG or includeFiles to ["/katex.cssWRONG"], no error is reported.