fix(proxy): handle error when proxy itself errors

[chaejunlee]

Description

closes #13703

Add error handling to the point when the proxy itself errors.

Right now, the proxy expects that the res is never undefined or other falsy value but there are some cases when the res is.

When the res is falsy, the thrown error becomes the error of accessing undefined value, not the original error.

This makes it hard for developers to trace the original error.

Therefore, I added a logic to handle error case when the res is falsy.

Additional context

My particular case of the problem was passing server.proxy's target as falsy value (null or undefined).

import { defineConfig } from 'vite';

// https://vitejs.dev/config/
export default defineConfig({
  server: {
    proxy: {
      '^/undefined': {
        target: undefined, // this one
      },
      '^/null': {
        target: null, // and this one
      },
    },
  },
});

AS-IS

Just seeing the error above, it is hard to track where the error started from.

TO-BE

The above error can give developers more specific information about where the error started from.

I believe that my fix can cover so much more errors. Due to the fact that it is caused by an incorrect expectation of the type of res.

---

What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

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[sapphi-red]

Since httpServer can be null (For example, if Vite is running in middleware mode), I think it would be better to output the error here directly (like we do below).

[chaejunlee]

Thank you for pointing it out!

I will change it right away.

[chaejunlee]

I have changed it to config.logger.error to output the error, and removed the httpServer error handling.

Now, the error console looks like this.

[sapphi-red]

Thanks! LGTM