[shell] do not return error if it was a user shell command or script

[savil]

Summary

For devbox shell, we do NOT return an exit error if a user's shell command had
returned a non-zero exit code. devbox cloud shell inherits this behavior.

For devbox run, we do return an error for the above scenario (as before).

How was it tested?

scenarios:

• verified local devbox shell
• cloud: custom built a binary and deployed a machine. Observed that we no longer
  return an error if the user's last command was errant:

cloud shell output:

(cloud) ❯ which foobar

(cloud) ❯ exit
exit
Shared connection to 9080557a662987.vm.devbox-vms.internal closed.

❯ echo "$?"
0

[savil]

Current dependencies on/for this PR:

• main
  • PR [shell] do not return error if it was a user shell command or script #489 👈

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[LucilleH]

ah, now I understand why we return nil prior to these changes