chore(css): catch postcss config error (fix #2793)

[sapphi-red]

Description

This PR improves error message of postcss config error.

• before

[vite] Internal server error: Unexpected identifier
  Plugin: vite:css
  File: D:/documents/GitHub/vite/packages/playground/css/inlined.css?inline
      at Object.compileFunction (node:vm:352:18)
      at wrapSafe (node:internal/modules/cjs/loader:1032:15)
      at Module._compile (node:internal/modules/cjs/loader:1067:27)
      at Object.Module._extensions..js (node:internal/modules/cjs/loader:1157:10)     
      at Module.load (node:internal/modules/cjs/loader:981:32)
      at Function.Module._load (node:internal/modules/cjs/loader:822:12)
      at Module.require (node:internal/modules/cjs/loader:1005:19)
      at require (node:internal/modules/cjs/helpers:102:18)
      at Object.search (D:\documents\GitHub\vite\packages\vite\dist\node\chunks\dep-7436056c.js:21010:43)
      at resolvePostcssConfig (D:\documents\GitHub\vite\packages\vite\dist\node\chunks\dep-7436056c.js:30232:22)

• after

[vite] Internal server error: Failed to load PostCSS config (searchPath: D:/documents/GitHub/vite/packages/playground/css): [SyntaxError] Unexpected identifier     
D:\documents\GitHub\vite\packages\playground\css\postcss.config.js:15
function testDirDep() {
         ^^^^^^^^^^
SyntaxError: Unexpected identifier
    at Object.compileFunction (node:vm:352:18)
    at wrapSafe (node:internal/modules/cjs/loader:1032:15)
    at Module._compile (node:internal/modules/cjs/loader:1067:27)
    at Object.Module._extensions..js (node:internal/modules/cjs/loader:1157:10)       
    at Module.load (node:internal/modules/cjs/loader:981:32)
    at Function.Module._load (node:internal/modules/cjs/loader:822:12)
    at Module.require (node:internal/modules/cjs/loader:1005:19)
    at require (node:internal/modules/cjs/helpers:102:18)
    at Object.search (D:\documents\GitHub\vite\packages\vite\dist\node\chunks\dep-55d87d37.js:21010:43)
    at resolvePostcssConfig (D:\documents\GitHub\vite\packages\vite\dist\node\chunks\dep-55d87d37.js:30232:22)
  Plugin: vite:css
  File: D:/documents/GitHub/vite/packages/playground/css/inlined.css?inline

close #2793

Additional context

---

What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

Before submitting the PR, please make sure you do the following

• Read the Contributing Guidelines.
• Read the Pull Request Guidelines and follow the Commit Convention.
• Check that there isn't already a PR that solves the problem the same way to avoid creating a duplicate.
• Provide a description in this PR that addresses what the PR is solving, or reference the issue that it solves (e.g. fixes #123).
• Ideally, include relevant tests that fail without this PR but pass with it.