Add more details to git clone errors

[benjaoming]

Fix for #2496

Before:

Now:

[benjaoming]

Build will fail until #2499 is merged.

[benjaoming]

Rebased

[ericholscher]

Thanks :)

[benjaoming]

@ericholscher hope it's okay to ask this pretty basic im-a-new-developer question... what's the process of deploying merged changes to readthedocs.org?

[ericholscher]

It will get merged into the rel branch and then deployed. We normally do about 1 deploy a week, so will likely get pushed up in a couple days.