Use library "Logs" for printing messages

[eponier]

https://erratique.ch/software/logs

[eponier]

It should allow to make things cleaner and nicer.

[vbgl]

The purpose of such a change is not only to make things cleaner. There are some situations (e.g., when the compiler is run in a web browser after compilation through js_of_ocaml) in which there is no “stderr”: there, a compiler warning simply crashes the compiler (and aborts compilation), which is not the purpose of a warning.