(pup-640) Update report schema for error events

Previously, special handling was added to synthesize a report
event even when the property application failed early. However,
when this change was made the report schema wasn't updated,
so the schema did not allow for several fields in the event
to be null (as they are for one of these synthesized events).

This change updates the report schema to reflect that certain
fields can be null. It also adds a spec test including an error
event, which requires the schema change in order to pass.
Merge pull request #2307 from kylog/issue/pup-640/report-schema-updates-for-error-events

(pup-640) Update report schema for error events
Merge branch 'master' of https://github.com/puppetlabs/puppet