Use set_backtrace instead of @backtrace in ActionView error

This pull req fixes handling of backtraces of `ActionView::Template::Error`.

`ActionView::Template::Error` wraps some other exceptions and delegates its backtrace to the original ones.
However, backtrace in `ActionView::Template::Error` is an instance variable and the variable overwrites `Exception#backtrace`.
It may cause unexpected troubles such as ruby/ruby#434.
So I introduce `set_backtrace` instead of `@backtrace` for `ActionView::Template::Error`.