Uniformize display of failed group updates

Standard and custom bug group actions did not provide the same user
experience when displaying error messages. bug_actiongroup.php displays
errors in a table, while bug_actiongroup_ext.php prints them with

elements.

This commit refactors the display code on both pages, by moving it to a
new API function bug_group_action_print_results().

Also fixes an issue with the page footer, which was displayed behind the
sidebar due to invalid HTML.

Fixes 0024643, 0024644