[BUG] Can start non-existent process

[BenElgar]

If a process doesn't exist we should throw an error rather appearing to succeed:

❯ exo start non-existent-process; echo $?
Job URL: http://localhost:44643/#/jobs/4k3e4914xqck3y47w5grwdtwkw
starting
0

[brandonbloom]

Related: #467