Better error message for missing let

I've run into this problem a number of times. The original error message
is actually a `NoMethodError` and it's not always immediately clear what
the solution is, particuarly if you're new to rspec or rswag

Wanted to see if this kind of behavior is something that the rswag team
would be interested in adopting. I have a few other of these in mind
(will do as small PRs, with tests)

rswag-specs/spec/rswag/specs/request_factory_spec.rb

@@ -160,6 +160,21 @@ module Rswag
             end
           end
 
+          context 'missing body parameter' do
+            before do
+              metadata[:operation][:parameters] = [{ name: 'comment', in: :body, schema: { type: 'object' } }]
+              allow(example).to receive(:comment).and_raise(NoMethodError, "undefined method 'comment'")
+              allow(example).to receive(:respond_to?).with(:'Content-Type')
+              allow(example).to receive(:respond_to?).with('comment').and_return(false)
+            end
+
+            it 'uses the referenced metadata to build the request' do
+              expect do
+                request[:payload]
+              end.to raise_error(Rswag::Specs::MissingParameterError, /Missing parameter comment/)
+            end
+          end
+
           context 'form payload' do
             before do
               metadata[:operation][:consumes] = ['multipart/form-data']