Source

File: wp-admin/includes/class-wp-ajax-upgrader-skin.php

public function get_error_messages() {
		$messages = array();

		foreach ( $this->errors->get_error_codes() as $error_code ) {
			$error_data = $this->errors->get_error_data( $error_code );

			if ( $error_data && is_string( $error_data ) ) {
				$messages[] = $this->errors->get_error_message( $error_code ) . ' ' . esc_html( strip_tags( $error_data ) );
			} else {
				$messages[] = $this->errors->get_error_message( $error_code );
			}
		}

		return implode( ', ', $messages );
	}