Allow issue edit exceptions to choke nicely
The attached patch allows issues raised by controller_issues_edit_before_save and controller_issues_edit_after_save to choke nicely, instead of bringing the user to an intimidating grey page.
The purpose of this is so that a plugin can use the controller_issues_edit_before_save hook and deny editing on specific conditions (e.g. an issue can't have it's assigned_to revoked after it's been set once, specific roles can't remove due dates, etc). Without this modification, plugins can't stop issues from saving.