Improve error handling across build pipeline

[scode2277]

Problem

When vocs build fails, it's extremely difficult to diagnose what went wrong. Errors are either silently swallowed, missing file path / line context, or immediately kill the process while discarding diagnostic info that Vite/Rollup already has.

This makes debugging build failures a trial-and-error process of commenting out pages until the broken one is found.

Current behavior

1. src/vite/build.ts: Prerender and scripts phases silently swallow errors

The catch blocks at lines ~59-61 (prerender) and ~82-84 (scripts) call the lifecycle hook but do not re-throw. When used without CLI hooks (e.g. programmatic API), the build silently continues after failure:

// prerender phase: error is caught but never re-thrown
catch (error) {
  hooks?.onPrerenderEnd?.({ error: error as Error })
  // ← build continues as if nothing happened
}

Same pattern for the scripts phase.

2. src/vite/prerender.tsx: No per-route error handling

The prerender loop has no try/catch per route. If a single page fails to render, the entire prerender aborts with no indication of which route caused the failure:

for (const route of routes) {
  const body = await mod.prerender(route)  // ← one bad page kills everything
  // ...
}

3. src/cli/commands/build.ts: Error details are discarded

The CLI hooks only print error.message and immediately process.exit(1), discarding the stack trace, source file path, and line/column info that Vite/Rollup errors carry:

onBundleEnd({ error }) {
  if (error) {
    spinner.client.fail(`bundles failed to build: ${error.message}`)
    process.exit(1)  // ← error.id, error.loc, error.frame, error.stack all lost
  }
}

Vite/Rollup errors typically include error.id (file path), error.loc (line/column), error.frame (code snippet), and error.plugin, none of which are surfaced.

4. src/vite/plugins/mdx.ts: No error wrapping around MDX compilation

The @mdx-js/rollup plugin is passed through with zero wrapping. MDX syntax errors (unclosed tags, invalid JSX, bad frontmatter) produce errors from micromark with file: '' (empty string), so the source file path is lost entirely. The error shape looks like:

{
  line: 28,
  column: 2,
  file: '',           // ← empty, useless
  reason: 'Unexpected end of file before name...',
  source: 'micromark-extension-mdx-jsx',
  ruleId: 'unexpected-eof'
}

The file path is available in the Rollup transform hook's id parameter but is never attached to the error.

5. src/vite/plugins/dev.tsx: Dev server SSR middleware has no catch block

The try { ... } finally { next() } pattern means SSR render errors crash the dev server instead of showing Vite's error overlay. Related: #163.

Expected behavior

• Build errors should include the file path, line/column (when available), and a readable error message
• Prerender/scripts phase errors should not be silently swallowed
• Dev server should show an error overlay instead of crashing
• The CLI should print Vite/Rollup error properties (error.id, error.loc, error.frame) when available

Reproduction

1. Create any MDX page with a syntax error, e.g. a lone < on a line:

   # Test page
   
   Some content
   
   <

2. Run vocs build
3. Output shows a huge output, too generic and not understandable by both human and AI scan
4. The only way to find the broken file is to binary-search by removing pages and retest the build

Environment

• vocs: 1.2.2 (also verified against 1.4.1 source - same issues exist)

I'm happy to submit a PR addressing these.