bug#32018: Guixsd pull error on very old install

[swedebugia]

Hi
Trying to get an updated an old GuixSD installation via guix pull on a pre 0.10 GuixSD returned an error message about gexp-modules not found. (details can be provided if asked). 

I suggest we either test pulling from old versions (and fix errors) or make it clear in the manual that pulling from very old GuixSD (older than 0.12?) is not supported.

Stating this clearly help people know what to expect.

What do you think? 
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Cheers Swedebugia