Bug Report

• Yes, I reviewed the contribution guidelines.
• Yes, more specifically, I reviewed the guidelines on how to write clear bug reports.

Describe the current, buggy behavior

When --version=x.y.z is supplied to theme update the theme is first deleted, then installed. This is disastrous if WP cannot find the specified version for a theme or if a user supplies an incorrect version as the theme has now been deleted from the site.

Problem code

		if ( isset( $assoc_args['version'] ) ) {
			foreach ( $this->fetcher->get_many( $args ) as $theme ) {
				$r = delete_theme( $theme->stylesheet );
				if ( is_wp_error( $r ) ) {
					WP_CLI::warning( $r );
				} else {
					$assoc_args['force'] = true;
					$this->install( array( $theme->stylesheet ), $assoc_args );
				}
			}
		}

Reference: Theme_Command.php 658-668

Describe how other contributors can replicate this bug

wp --debug theme update twentynineteen --version=7.2
# Reports Error: Can't find the requested theme's version 7.2 in the WordPress.org theme repository (HTTP code 404).
wp --debug theme update twentynineteen --version=7.2
# Reports Warning: The 'twentynineteen' theme could not be found.
wp theme list
# twentynineteen now omitted from list

Describe what you would expect as the correct outcome
Theme update fails safely and does not remove the existing theme.

OS:     Linux 3.10.0-1062.18.1.el7.x86_64 #1 SMP Tue Mar 17 23:49:17 UTC 2020 x86_64
Shell:  /bin/bash
PHP binary:     /.socket/php/usr/bin/php
PHP version:    7.4.11
php.ini used:   /etc/php.ini
MySQL binary:   /usr/bin/mysql
MySQL version:  mysql  Ver 15.1 Distrib 10.3.22-MariaDB, for Linux (x86_64) using readline 5.1
SQL modes:      STRICT_TRANS_TABLES,ERROR_FOR_DIVISION_BY_ZERO,NO_AUTO_CREATE_USER,NO_ENGINE_SUBSTITUTION
WP-CLI root dir:        /var/www/wp-cli
WP-CLI vendor dir:      /var/www/wp-cli/vendor
WP_CLI phar path:
WP-CLI packages dir:
WP-CLI global config:   /home/benchmark-user/.wp-cli/config.yml
WP-CLI project config:
WP-CLI version: 2.5.0-alpha

Provide a possible solution
Rename theme, apply update, then delete or rename directory depending upon install().