Fixed that nested transactions now work by letting the outer most transaction have the responsibilty of starting and rolling back the transaction. If any of the inner transactions swallow the exception raised, though, the transaction will not be rolled back. So always let the transaction bubble up even when you've dealt with local issues. Closes #231 and #340.

git-svn-id: http://svn-commit.rubyonrails.org/rails/trunk@242 5ecf4fe2-1ee6-0310-87b1-e25e094e27de