[PATCH v5] copy: distinguish "file exists" cases and add a hint (BC)

[timeless]

Kevin Bullock wrote:
> I'm not totally clear on what our approach to translations during a freeze is either. I'm going by the freeze rules on the wiki, and the fact that translation always happens on stable anyway, both of which lead me to believe it should be okay?

We should probably clarify this.

Note that in other projects the normal approach is that at freeze,
conceptually all strings are sent to localizers, who can then take the
project-as-is-at-freeze-time and update translations, and then
asynchronously transmit their updates.

This means that if I change canonical, I've broken the translation system.

There are a number of ways to handle this, but it's worth
understanding (and documenting) how freezes should work for a project.

Pierre-Yves David wrote:
> Okay, I don't have a too strong opinion on this and I trust you will take
> the right decision.

> Worse case will have user or translator (hopefully) complaining and we'll adjust.

Sorry for not getting back to this quickly, fwiw, hoping translators
will complain is close to
{pick-any-expression-describing-impossible-wishes}.