Return Gandi error message for Gandi LiveDNS domains GET request

[HLFH]

Precise the Gandi error message for Gandi LiveDNS domains GET request instead of "Unable to get base domain [...]".
You could get error messages such as:

• access was denied to this resource ;
• the resource could not be found ;
• and maybe a few others.

And these error messages pinpoint the user and the project contributors to the right direction.
This PR is a good start that will help contributors to document each error that the user can get when certbot-plugin-gandi is unable to get the base domain.