Display the DBAL exception when an error occurs

fixed #80 

I've decided to take an easier approach which will work for any random layer

I could try and find out the code for postgresql, but there's still many drivers to take into account. I don't think it's managable.

If this one ever makes it, perhaps we can take another shot:
http://www.doctrine-project.org/jira/browse/PHPCR-71