creusot-setup: Print issues on stderr instead of stdout when bailing

[Lysxia]

@xldenis that would provide a more informative error message in #1294 (comment)

[xldenis]

Testing creusot-contracts ... failure
Error: Why3 has version 1.7.2, expected version is 1.8.0
Error: why3find has version 1.0.0, expected version is 1.1.1
Error: Please run 'cargo creusot setup status' to diagnostic and fix the issue(s)
thread 'main' panicked at creusot/tests/creusot-contracts.rs:120:9:
output of creusot-contracts failed
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

ah! opam strikes again.