svelte-kit dev crashes on page load if there is a syntax error in a layout component

[Conduitry]

Describe the bug
npm run dev/svelte-kit dev crashes on page load if there was a syntax error in the layout component when you started it.

Logs

/.../foo/node_modules/.pnpm/svelte@3.37.0/node_modules/svelte/compiler.js:16752
            const error = new CompileError(message);
                          ^

CompileError [ParseError]: Unexpected token
    at error (/.../foo/node_modules/.pnpm/svelte@3.37.0/node_modules/svelte/compiler.js:16752:20)
    at Parser$1.error (/.../foo/node_modules/.pnpm/svelte@3.37.0/node_modules/svelte/compiler.js:16828:10)
    at Parser$1.acorn_error (/.../foo/node_modules/.pnpm/svelte@3.37.0/node_modules/svelte/compiler.js:16822:15)
    at Object.read_script [as read] (/.../foo/node_modules/.pnpm/svelte@3.37.0/node_modules/svelte/compiler.js:7501:17)
    at tag (/.../foo/node_modules/.pnpm/svelte@3.37.0/node_modules/svelte/compiler.js:15887:34)
    at new Parser$1 (/.../foo/node_modules/.pnpm/svelte@3.37.0/node_modules/svelte/compiler.js:16787:22)
    at parse$3 (/.../foo/node_modules/.pnpm/svelte@3.37.0/node_modules/svelte/compiler.js:16919:21)
    at compile (/.../foo/node_modules/.pnpm/svelte@3.37.0/node_modules/svelte/compiler.js:29949:18)
    at compileSvelte (/.../foo/node_modules/.pnpm/@sveltejs/vite-plugin-svelte@1.0.0-next.5_svelte@3.37.0+vite@2.1.5/node_modules/@sveltejs/vite-plugin-svelte/dist/index.js:291:48)
    at TransformContext.transform (/.../foo/node_modules/.pnpm/@sveltejs/vite-plugin-svelte@1.0.0-next.5_svelte@3.37.0+vite@2.1.5/node_modules/@sveltejs/vite-plugin-svelte/dist/index.js:679:33) {
  code: 'parse-error',
  start: { line: 5, column: 0, character: 34 },
  end: { line: 5, column: 0, character: 34 },
  pos: 34,
  filename: '/.../foo/src/routes/$layout.svelte',
  frame: '3: \n4: +\n5: </script>\n   ^\n6: \n7: <slot />',
  plugin: 'vite-plugin-svelte',
  id: '/.../foo/src/routes/$layout.svelte',
  pluginCode: "<script>\n\timport '../app.css';\n\n+\n</script>\n\n<slot />\n",
  loc: {
    file: '/.../foo/src/routes/$layout.svelte',
    line: 4,
    column: 3
  }
}

To Reproduce
Edit src/routes/$layout.svelte to contain a syntax error that will cause an exception in the Svelte compiler. Start Kit in dev mode, and load the page in the browser.

Expected behavior
This should display some sort of 500 response in the browser.

Severity
Low-ish, but annoying.

Additional context
Syntax errors introduced after starting npm run dev in src/routes/$layout.svelte don't seem to be a problem, nor do syntax errors in other routes, even if they were present when starting the dev server.

[Rich-Harris]

I suspect this will be fixed as a side-effect of #626

[Conduitry]

I think I recall trying this again after #626 and seeing that it was now fixed, but never closed this issue apparently.