Return ResourceNotFoundException when failing to open a git repository

If we attempt to open a repository that does not exist, the exception
thrown is eventually wrapped into a servlet exception with code 500.
This is incorrect, as the response should actually be a 404.

The first step therefore is to handle the exception by wrapping it
into a rest ResourceNotFoundException.

Bug: Issue 16730
Change-Id: If87f717012bf7f22ae31b12822e5453c711e3cdc