Make `reserved_path` section mandatory in routes
Make `reserved_path` section mandatory in routes

Pen testing on staging triggered a few thousand 500 errors when making
requests to `/paths`. This change means a request to `/paths` will get a
400 Bad Request response.

alphagov/url-arbiter,chrisroos/url-arbiter,gds-attic/url-arbiter,gds-attic/url-arbiter,gds-attic/url-arbiter,chrisroos/url-arbiter,alphagov/url-arbiter,alphagov/url-arbiter,chrisroos/url-arbiter