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>
Merge pull request #344 from dachary/wip-9842-deep-merge

display informative error message if schedule fails to deep_merge