nuageinit; Skip SSH keys and doas/sudo if adduser fails

[emaste]

Summary

Previously we would still try to add SSH keys after a failed adduser,
which would result in an uncaught Lua error, causing the remainder of
the setup to be skipped.

Reported by:    llm-postcr

llm-postcr in response to eca5b28692eac5ae597eff6a13629c17c1e40ace

[dtxdf]

LGTM!