trivial: Remove broad exceptions where possible

trivial: Remove broad exceptions where possible

The are two somewhat significant changes:

* The behavior of 'Bundle.add_patch' is changed. Previously this would
  raise an exception if the provided patch already existed in the
  bundle. Since this code was only used in one location, change this to
  return the BundlePatch if valid else None and change the calling code
  to check return value instead of catching the exception.
* Use a context manager to open the config file in pwclient. This loses
  a little granularity in error messaging, but this is a worthy
  compromise.

Signed-off-by: Stephen Finucane <stephenfinucane@hotmail.com>
Reviewed-by: Daniel Axtens <dja@axtens.net>