posts: fix exception when tagging post with char:copy:foo.

Fixup for 015c6dc7d. Show a warning about failure to add a tag instead
of raising an exception when trying to tag a post with `char:copy:foo`.
This tries to create a tag named `copy:foo` then set the category to
character, which doesn't work because `copy:foo` isn't a valid tag name.

app/models/tag.rb

@@ -197,7 +197,7 @@ class Tag < ApplicationRecord
         tag = find_or_create_by(name: normalize_name(name))
 
         if category.present? && current_user.present? && Pundit.policy!(current_user, tag).can_change_category?
-          tag.update!(category: categories.value_for(category))
+          tag.update(category: categories.value_for(category))
         end
 
         tag