keep the original exception's backtrace

The StatementInvalid exception keeps the original exception's message, but not the backtrace.  So, we end up getting a message and backtrace like:

```
ArgumentError: wrong number of arguments (1 for 2)

.../vendor/gems/1.9.3/ruby/1.9.1/gems/activerecord-2.3.14.github25/lib/active_record/connection_adapters/abstract_adapter.rb:228:in `rescue in log'
.../vendor/gems/1.9.3/ruby/1.9.1/gems/activerecord-2.3.14.github25/lib/active_record/connection_adapters/abstract_adapter.rb:205:in `log'
```

I want to know where that ArgumentError came from.