Bug 22575: Item type administration uses invalid error class for dialog

Bug 22575: Item type administration uses invalid error class for dialog

This patch modifies the item types administration script so that it
passes a valid message type to the template, where the type is used as a
CSS class. "Error" is a nonexistent dialog class. It should be "alert."

To test, apply the patch and go to Administration -> Item types.

 - Try to delete an item type which is in use. The style of the error
   dialog should be correct.
 - Try to add an item type using a code which already exists. The error
   should look correct.

Signed-off-by: Pierre-Marc Thibault <pierre-marc.thibault@inLibro.com>
Signed-off-by: Bin Wen <bin.wen@inlibro.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>