release_config: various cleanup

- Allow scl and textproto data to coexist for now
- Print warnings to stderr instead of stdout.
- Improve formatting of output
- Set displays the new value for all configs, and what file changed.
- Use prettier error messages for better UX
- put build-flag on the path.

Bug: 328495189
Test: manual
Change-Id: I4c38860c2fb24db5111e0cecf790660a4ff2b8b2