switch from Prelude.catch to Control.Exception.catch (fix for ghc-7.6)

This fixes the package for ghc-7.6.

I am unexperienced when it comes to git branches. I was working on something else (prepared statements) in the master branch of my fork, so I created a branch based on the current version from hdbc/hdbc-postgresql called "ghc-7.6-fix". I hope this is ok and I did everything correctly.