nanobsd: Add an err() convenience function

[senguptaangshuman17_gmail.com]

Summary

Add err() to echo a message to stderr and exit with the NanoBSD
fatal error code. Use it in the three existing echo and exit sites.

[senguptaangshuman17_gmail.com]

No code changes in this commit. An earlier commit in the stack (D58931) needed a commit-message reword per review feedback, which required a bottom-up arc diff refresh of every commit above it to keep the stack applying cleanly. That refresh is what reset this revision's accepted status, not a content change.

@jlduran, could you re-accept when you get a chance?