silent failure when invalid custom_services_dir specified

[GoogleCodeExporter]

What steps will reproduce the problem?
1. set custom_services_dir to one of the following invalid directory names
   ('addons', 'api', 'bsd', 'emane', 'misc', 'netns',
    'phys', 'services', 'xen')
2. an error is only printed if 'services' is used
3. any services in the given directory will fail to load

What is the expected output? What do you see instead?

Error message should be printed.

When a custom services directory such as "/home/username/.core/myservices" is 
used, core-daemon will update sys.path to include "/home/username/.core" and 
attempt to exec "from myservices import *".

The list of invalid directories is from the directories in the 
core/daemon/core/ directory.

Original issue reported on code.google.com by ahrenh...@gmail.com on 30 Apr 2014 at 9:13