fix to user journey for 'report a problem' errors

before this change, a 500 (or any non-422) error status
from the feedback app would leave the user hanging without
an error message. After this change, the error message
from the server is properly displayed.
Merge pull request #153 from alphagov/js_fix_for_report_a_problem_serverside_errors

fix to user journey when 'report a problem' server-side errors occur