(maint) Clearer error message to reflect origin.

Zachary ran across a case where this error message was being thrown up
and it wasn't immediately clear that it occured during the collection of
exported resources, due to references to local resources.  Make this
clearer for future cases.
(maint) Include information about how PRs are handled

Our process and expectations around how pull requests are handled hasn't
been the most clear. This causes problems not only for those submitting
pull requests, but also for those handling them. This tries to lay some
ground rules.
Merge branch 'cleaner-error'

* cleaner-error:
  (maint) Clearer error message to reflect origin.
Merge branch 'master' of https://github.com/puppetlabs/puppet