fix: assert provider retrieved successfully before using link

Fixes: #296

Author: bethesque

lib/pact/hal/entity.rb

@@ -23,6 +23,10 @@ def get(key, *args)
         _link(key).get(*args)
       end
 
+      def get!(key, *args)
+        _link(key).get!(*args)
+      end
+
       def post(key, *args)
         _link(key).post(*args)
       end

lib/pact/provider/verification_results/publish.rb

@@ -135,7 +135,7 @@ def provider_name
         end
 
         def provider_entity
-          @provider_entity ||= pact_entity.get(PROVIDER_RELATION)
+          @provider_entity ||= pact_entity.get!(PROVIDER_RELATION)
         end
       end
     end