bad error reporting for improper headers

[dherman]

When an .md file has improperly indented headers it seems to crash the JS code and report an internal error. For example, running codex on a project with a file containing the following:

---
title: Home

---

Hello, world.

produces the error:

/Users/dherman/Sources/tc39-codex-wiki/node_modules/codex/node_modules/yaml/lib/yaml.js:179
  throw new Error(msg + ', ' + context(this.peek()[1].input))
        ^
Error: document not properly dedented, near "\n---"
    at Parser.expect (/Users/dherman/Sources/tc39-codex-wiki/node_modules/codex/node_modules/yaml/lib/yaml.js:179:9)
    at Parser.parseDoc (/Users/dherman/Sources/tc39-codex-wiki/node_modules/codex/node_modules/yaml/lib/yaml.js:266:8)
    at Parser.parse (/Users/dherman/Sources/tc39-codex-wiki/node_modules/codex/node_modules/yaml/lib/yaml.js:232:19)
    at Object.eval (/Users/dherman/Sources/tc39-codex-wiki/node_modules/codex/node_modules/yaml/lib/yaml.js:384:46)
    at /Users/dherman/Sources/tc39-codex-wiki/node_modules/codex/lib/codex/plugins/pages.js:33:26
    at Array.forEach (native)
    at /Users/dherman/Sources/tc39-codex-wiki/node_modules/codex/lib/codex/plugins/pages.js:16:13
    at /Users/dherman/Sources/tc39-codex-wiki/node_modules/codex/lib/codex/plugins/pages.js:79:29
    at /Users/dherman/Sources/tc39-codex-wiki/node_modules/codex/lib/codex/plugins/pages.js:79:29
    at Object.oncomplete (/Users/dherman/Sources/tc39-codex-wiki/node_modules/codex/lib/codex/plugins/pages.js:85:27)

Dave

[logicalparadox]

This is the default yaml module error. Will surround it with a try/catch so it can report which markdown file it is referring to.