General: Prevent php warning for not readable files

[Ninos]

Having unreadable php-files in theme-directory triggers following php-warning:

file_get_contents(/path/to/theme//maySubdir/file.php): Failed to open stream: No such file or directory in /wp-includes/class-wp-theme.php#L1333

This PR hardens the get_post_templates() function a bit.

Trac:
https://core.trac.wordpress.org/ticket/63743

[github-actions]

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[mindctrl]

How would site admins, or log monitoring tools, know when there's a file read error if we suppress errors?

[github-actions]

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[Ninos]

How would site admins, or log monitoring tools, know when there's a file read error if we suppress errors?

For me such error output only makes sense for files which must be readable. The function parses all php-files in theme's subdirectories, not only template files, also which are not relevant for templating. Would be better to have some predefined folders (theme dir, /templates, ...) for parsing (also for performance reasons), but this would may break compatibility with some themes.