Issue #2220: propdel returns success on deleting a non-existent prop.

   * subversion/libsvn_wc/props.c
     (svn_wc_prop_set2): Modified with check to see
     if property already exists. Check done only if 
     skip_checks is not TRUE.

   * subversion/clients/cmdline/propdel-cmd.c
     (svn_cl__propdel): Modified to always pass TRUE
     for the skip_checks parameter of svn_client_propset2().

   * subversion/tests/clients/cmdline/prop_tests.py
     (remove_nonexistent_prop): Added function
     to test for error on removal of non-existent property.

   * subversion/tests/clients/cmdline/prop_tests.py
     (test_list): Added function with Skip().