Add a user-friendly error message when reconfig fails in condor_ce_config_generator (SOFTWARE-1705)

This also fixes a Python 2.4 incompatibility (since
subprocess.CalledProcessError did not exist in Py2.4)
Add filterResources to filter resource ads against given constraints

This makes it easier to use this module as a library, since it allows us
to filter resources without having to provide the constraints as
command-line arguments.
Remove query ad-related code

The query ad is redundant given the other filtering options present in
the code, and the other options are more flexible.
Change Resource into ResourceAd

Remove the redundant resource_ad member from the Resource class and
simply make the Resource class a subclass of classad.ClassAd itself;
then rename it to ResourceAd.