Add pdf Format Handling For render_respond_to_format_with_error_message

[aaronskiba]

Please complete the following fields as applicable:

What version of the DMPRoadmap code are you running? (e.g. v2.2.0)

• 4.0.2+portage-4.0.3
• app.rollbar.com item entry

Expected behaviour:

• Assuming plan.id = 99999 doesn't exist in the db, then the url path /plans/99999 should return the following:
  

• One might expect the same behaviour when /plans/99999/export.pdf?export%5Bquestion_headings%5D=true is requested

Actual behaviour:

• An ActionController::UnknownFormat error is encountered instead
  

[aaronskiba]

ActionController::UnknownFormat is also raised when a user attempts to download an unauthorised plan within '%{org_title} Plans' (see #732 for more details).