Fix NameError traceback when checking .treeinfo.signed

If you want e.strerror, you gotta do 'except ... as e:'.
Minor --instrepo cleanup

Nobody does --addrepo REPOID=URL --instrepo REPOID, they all do
--instrepo URL. So let's document it as such.

Also, change the name/id of the repo given with --instrepo to
'instrepo', which is sensible, right?
Add --instrepokey

This lets testers define the key that will be used to verify the
contents of instrepo when they're using a custom instrepo.