5017: Fix bug if x509_proxies_plugin is not defined

If no credentials are defined in the frontend, an exception was thrown
due to an undefined variable. This patch defines the variable and emits
an error message.
5017: Don't index into an empty list
Merge branch 'v3/5017' into branch_v3_2
Merge branch 'branch_v3_2' of ssh://cdcvs.fnal.gov/cvs/projects/glideinwms into branch_v3_2