gracefully handle errors with checkouts

in order to keep quelpa from aborting init if there is a problem with checking out (like when having no internet connection) I added some stuff...

the plan is to implement a behavior like this:

- melpa cannot be checked out and is not already there: do nothing at all, print a message
- melpa cannot be updated: continue as normal, print a message
- a package checkout cannot be updated: use the existing checkout, print a message
- a package cannot be checked out: do nothing, print a message