Change in crm_get_group_contacts behavior on error

I'm including a patch to make crm_get_group_contacts behave like crm_fetch_contact does on error: rather than call fatal() and make a non-local exit, I've changed it to return an error object.

Patch has two components:
1) CRM/Core/Error.php now has a static create_error() method that takes the same arguments as the HL API call _crm_error(). It does exactly the same thing: create an error and put it on the error stack.

2) CRM/Contacts/BAO/GroupContacts.php now calls create_error instead of fatal in the two places where fatal was called before.

This creates the possibility of the calling application to deal with a missing gid or lack of permissions more elegantly than is currently possible.