Does not verify whether the MySQL client is installed

[gwolf]

Bee has several commands that require having the MySQL client, which is not a system requirement for Backdrop. It can lead to frustrating errors, such as:

/var/www/bkdrp_proglo $ bee sqlc
sh: 1: mysql: not found
✓ gwolf@backdrop『33』/var/www/bkdrp_proglo $ bee db-drop
Are you sure you want to drop the contents of bkdrp_proglo ? (y/N): y
sh: 1: mysql: not found
sh: 1: mysql: not found

 [✔] The 'bkdrp_proglo' database was successfully dropped. 
 [✔] The 'bkdrp_proglo' database was successfully created. 

I will be submitting a pull request proposing a patch that fixes this by checking before exec() / proc_open().