Complain when file/http param resource does not exist

If a user provides an arg via file://, http://, https://
and that resource does not exist, we should immediately stop
and let them know they're specifying a resource that does not
exist.  Otherwise the user will generally get an error message
further downstream that makes it harder to troubleshoot.  Since
most of these file params are used for complex types, this generally
happens in the json parsing and/or the arg shorthand processing.
Fix py3 test bug

Still not sure why httpretty didn't like the previous version,
but this would intermittently fail on py3 (maybe 20% of the time).

Doing it this way, I'm not able to get the tests to fail after
running them repeatedly.
Merge branch 'error-file' into develop

* error-file:
  Fix py3 test bug
  Complain when file/http param resource does not exist