A Puppet report from an interrupted run can cause exit_status to throw an exception

Recently when playing around with reports in irb, I discovered that the exit_status method raises an exception when called against a report from an interrupted (ctrl + c or other) puppet run.

irb(main):007:0> a.exit_status

NoMethodError: undefined method `[]' for nil:NilClass

        from /Users/matthaus/src/puppet/lib/puppet/transaction/report.rb:380:in `exit_status'

        from (irb):7

        from /usr/local/opt/ruby/bin/irb:11:in `<main>'

The metrics instance variable is not fully populated for that report and the lookups performed in the exit_status method are not safe with respect to metrics being an empty hash.