Print warnings to stderr

When tutor is run as root, the `tutor config printroot` and `tutor
config printvalue` commands used to write to stdout, which was causing
issues when combined with `tutor config render` and `tutor local
settheme` commands.

See discussion:
https://discuss.overhang.io/t/error-while-install-indigo-theme/1056

CHANGELOG.md

@@ -4,6 +4,8 @@ Note: Breaking changes between versions are indicated by "💥".
 
 ## Unreleased
 
+- [Bugfix] Print warnings to stderr
+
 ## v11.1.4 (2021-02-04)
 
 - [Security] Apply security patch [26358](https://github.com/edx/edx-platform/pull/26358).

tutor/fmt.py

@@ -37,7 +37,7 @@ def question(text):
 
 
 def echo_alert(text):
-    echo(alert(text))
+    echo_error(alert(text))
 
 
 def alert(text):