Correctly stop further execution when autoloader is missing

[swissspidy]

Summary

This is a follow-up to #1234 / #2962. A small oversight that happened when refactoring that PR.

Basically, the goal is this:

1. When the autoloader is missing (e.g. ! class_exists( '\Google\Web_Stories\Plugin' ) === true) report error in both WordPress admin and the terminal.
2. When JS files are missing (e.g. ! file_exists( WEBSTORIES_PLUGIN_DIR_PATH . '/assets/js/edit-story.js' ) === true) report error in WordPress admin.
3. When the autoloader is missing stop further execution to prevent fatal errors, in both WordPress admin and the terminal.

Previously, the last step only worked in CLI context but not in WordPress admin.

This PR restores expected behavior.

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Fixes #3231

[github-actions]

Size Change: 0 B

Total Size: 1.08 MB

ℹ️ View Unchanged

Filename	Size	Change
assets/css/edit-story.css	269 B	0 B
assets/css/stories-dashboard.css	305 B	0 B
assets/css/web-stories-embed-block.css	515 B	0 B
assets/js/edit-story.js	470 kB	0 B
assets/js/stories-dashboard.js	541 kB	0 B
assets/js/web-stories-activation-notice.js	55.8 kB	0 B
assets/js/web-stories-embed-block.js	16.4 kB	0 B

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[swissspidy]

@spacedmonkey could you please review?

[spacedmonkey]

👍