Replace all trigger_error() with wp_trigger_error()

[MichelleBlanchette]

See https://developer.wordpress.org/reference/functions/wp_trigger_error/

PHPCS linting warnings never mentioned this function as the recommended alternative, but I just now happened upon it.

trigger_error() found. Debug code should not normally be used in production.
(WordPress.PHP.DevelopmentFunctions.error_log_trigger_error) phpcs

[MichelleBlanchette]

Marking this as a Bug since we've literally seen how triggering errors can actually break functionality in some cases per #226

[MichelleBlanchette]

Really, I'd more prefer all logging be sent to a custom log file which is visible in wp-admin per #134