nuageinit: Split pw line on :s

[emaste]

Summary

Previously we parsed a v7 pw string using "%a+:" in the match pattern,
which matches a string of letters preceeding a colon.  This would fail
for userids ending in a digit or _.  Instead, match runs of non-colons
using "[^:]*".

While here, correct the error string from an existing test.

adduser("plop") should fail because adduser requires a table, not a
string.  Report this as the failure reason, rather than claiming that
the faulre was due to an empty value.

Reported by:    llm-postcr
Sponsored by:   The FreeBSD Foundation

[emaste]

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