Template of shutdown handler for fatal errors should not be displayed for CLI scripts

[iandunn]

When running a CLI script that has a syntax error, the template's raw HTML and CSS are output to the command line.

I don't think that the shutdown handler for fatal errors should be loaded in a CLI context; instead, the default PHP behavior should be used instead.

This should handle the case where the CLI script is being run through WP-CLI, and also older scripts that were built before WP-CLI existed, which just load WP manually (i.e., check 'cli' === php_sapi_name() rather than defined( 'WP_CLI' ) ).

[ocean90]

An alternative would be to add a wp_die handler for CLI context.

[SergeyBiryukov]

Replying to ocean90:

An alternative would be to add a wp_die handler for CLI context.

We have (currently unused) _scalar_wp_die_handler(), which seems like a good fit.

Introduced in [20063], unused since [21818].

Trac ticket: https://core.trac.wordpress.org/ticket/47266