Improve error response in cfn Outputs validation

Relying on discussion in commit
I0d7e3e57e479ddb658645b8664bc847866180cda, this patch
improve error message for a better understanding of
error in Outputs and Resources sections.

Change-Id: I2a16093a57b474ef4636975b5fc0df4103678a08
Merge "Improve error response in cfn Outputs validation"