add some checks when downloading files

[maelle]

E.g. when downloading the template, try again a few times?

[maelle]

I broke this since the templates are now in the appendix of dev_guide... the best might be to have a regularly updated copy in inst/... I'm sorry!

Or in a central place that'd be read by dev_guide too?

[annakrystalli]

We fixed this in dev_guide (ie they are standalone files now) and I've updated the URL in pkgreviewr so I'll close this issue. 💪