Merged r12070 from trunk to 2.3-stable

not use assert_not_nil in Errors#[].

r7593 etc. replaced Rails2 Errors#on.
Rails3 Errors#[] always return array.
So, Rails3 Errors#[] is always not nil.

git-svn-id: svn+ssh://rubyforge.org/var/svn/redmine/branches/2.3-stable@12071 e93f8b46-1217-0410-a6f0-8f06a7374b81