suppress "stock file is absent, creating a new one" eprint

[nicbus]

Instantiating a runtime by calling Runtime::load_attach() currently prints stock file is absent, creating a new one.

This is inconvenient and I think this output should be suppressed.
Similarly to what's been done in #23, I propose to protect that eprint behind a feature flag (e.g. log, which appears otherwise unused).

[dr-orlovsky]

Agree