Report actual error in error message
Report actual error in error message

While debugging a YAML change just now, I found it impossible to work
out what jenkins-projects-checks.py was actually complaining about
without this additional information.  I thought others might benefit
from it too.

Change-Id: I79b8286a3ef74008e5b0927bddb60f9c98b9b629

Tesora/tesora-project-config,dongwenjuan/project-config,Tesora/tesora-project-config,openstack-infra/project-config,dongwenjuan/project-config,noorul/os-project-config,noorul/os-project-config,openstack-infra/project-config