template: When rendering exception from Jinja, include exception name.

This will give basic context or *what* the error is, otherwise there're
messages like:

fatal: [tcwg.ci.linaro.org] => {'msg': 'files/private/jenkins_ssh_publishers.xml.tcwg-ci', 'failed': True}

Which give a filename related to error, but not an idea what error is.

Addresses #7103.