fix(output): always report warnings on stderr

Signed-off-by: bircni <bircni@icloud.com>

docs/migration-1.0.md

@@ -298,7 +298,8 @@ of printing nothing. See `tea help formatting`.
 When stdout is not a terminal, lists print tab-separated values without header,
 color or links, with times in RFC 3339 UTC, and a `--page` hint goes to stderr;
 prefer `--json` in scripts. Commands that change resources report each change on
-stderr only on a terminal. Create commands print the URL of the new resource on
+stderr only on a terminal; warnings, such as a step that was skipped or failed,
+always go to stderr. Create commands print the URL of the new resource on
 stdout, so `url=$(tea issue create …)` works; other changes print nothing when
 piped. Bodies are rendered as Markdown only on a terminal (`tea config set
 markdown raw` turns this off). See `tea help formatting` for the terminal layout.