create-diff-object: better reachability error message

If a patch changes a single function which is in a special section that
we don't support, create-diff-object reports "no changed functions were
found".  Give a clearer error message in that case, by checking
reachability errors before unchanged errors and by printing all
reachability errors errors instead of the first one it encounters.

Fixes #150.
Merge pull request #151 from jpoimboe/cdo-error-order

create-diff-object: better reachability error message