Media: Correct error message for rejected image uploads

[benitoalba]

Summary

Plupload currently maps every image HTTP_ERROR to the image-processing message, including HTTP 4xx responses from hosting security layers. This makes a ModSecurity rejection look like an image-size or 2560-pixel failure.

This change uses the existing generic HTTP error for image uploads rejected with a 4xx response. Image/server failures outside the 4xx range retain the existing image-processing guidance, and non-image behavior is unchanged.

The patch does not alter filename sanitization or attempt to bypass the hosting firewall. A clean Core environment accepts test'image.jpg and stores it as testimage.jpg; the original host-specific ModSecurity rejection is not reproducible locally without intercepting the response.

Trac ticket: https://core.trac.wordpress.org/ticket/64617

Testing

• npm run test:e2e -- tests/e2e/specs/media-upload.test.js — 4 passed
• npm run grunt -- jshint:corejs — passed
• npm run typecheck:js — passed
• npm run grunt -- qunit:compiled — build completed, then Grunt's Windows npx launcher returned spawn EINVAL
• Direct underlying QUnit command, npx playwright test --config tests/qunit/playwright.config.js — 2 passed; 566/566 compiled and 566/566 source tests passed

The E2E coverage verifies a clean apostrophe-containing JPEG upload, an intercepted HTTP 403 response using the generic response message without 2560-pixel guidance, and an intercepted HTTP 500 response retaining the image-processing guidance.

Use of AI Tools

AI assistance: Yes
Tool(s): OpenAI Codex
Model(s): GPT-5.6 Sol, GPT-5.6 Luna
Used for: Planning, implementation, test authoring, local validation, and an independent final patch review.

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This Pull Request is for code review only. Please keep all other discussion in the Trac ticket. Do not merge this Pull Request. See GitHub Pull Requests for Code Review in the Core Handbook for more details.

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