Display better error when hooks already exist

When non-Overcommit hooks were already installed in a repo, Overcommit
would raise an exception. Change the behavior to gracefully catch the
exception and display a normal error message.

Change-Id: Ibc11c3d9f476a47c403133ebe1b90f138078a1ca
Reviewed-on: http://gerrit.causes.com/36003
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane@causes.com>