Move ErrorList to util

Part of ip-per-service
Let's see... Easiest is to just make errorListInternal public (called ErrorListErr?).

More extensive:

* Delete the ErrorList.ToError() function
* Remove errorListInternal, and move its Error() function to be on ErrorList
* Make a ```func ToError(errs []error) error``` function that returns nil if errs is nil, otherwise an ErrorList
* I don't feel bad about making folks pass around []error until they want to collapse into a single error, because ErrorList is not really a helpful type at the moment-- it's actually more typing than []error and doesn't offer any features besides ToError.