display informative error message if schedule fails to deep_merge

Display the two yaml structures that fail to merge in case of an
exception. That's a useful hint to find out what's wrong.

Signed-off-by: Loic Dachary <loic-201408@dachary.org>
Never use ```except:``` without specifying a class of exceptions to catch. A quick read here will explain why and what to do instead:

https://docs.python.org/2/howto/doanddont.html#except