Modify invalid Snippet response codes

Previously if a Snippet service had invalid params
it would return a 403 (forbidden) status code, but a
422 (Unprocessable Entity) is more appropriate in this scenario

app/services/snippets/base_service.rb

@@ -46,7 +46,7 @@ module Snippets
         snippet.errors.add(:snippet_actions, 'have invalid data')
       end
 
-      snippet_error_response(snippet, 403)
+      snippet_error_response(snippet, 422)
     end
 
     def snippet_error_response(snippet, http_status)

spec/services/snippets/create_service_spec.rb

@@ -313,6 +313,7 @@ RSpec.describe Snippets::CreateService do
       it_behaves_like 'creates repository and files'
       it_behaves_like 'after_save callback to store_mentions', ProjectSnippet
       it_behaves_like 'when snippet_actions param is present'
+      it_behaves_like 'invalid params error response'
 
       context 'when uploaded files are passed to the service' do
         let(:extra_opts) { { files: ['foo'] } }
@@ -340,6 +341,7 @@ RSpec.describe Snippets::CreateService do
       it_behaves_like 'creates repository and files'
       it_behaves_like 'after_save callback to store_mentions', PersonalSnippet
       it_behaves_like 'when snippet_actions param is present'
+      it_behaves_like 'invalid params error response'
 
       context 'when the snippet description contains files' do
         include FileMoverHelpers

spec/services/snippets/update_service_spec.rb

@@ -698,6 +698,7 @@ RSpec.describe Snippets::UpdateService do
       it_behaves_like 'when snippet_actions param is present'
       it_behaves_like 'only file_name is present'
       it_behaves_like 'only content is present'
+      it_behaves_like 'invalid params error response'
       it_behaves_like 'snippets spam check is performed' do
         before do
           subject
@@ -725,6 +726,7 @@ RSpec.describe Snippets::UpdateService do
       it_behaves_like 'when snippet_actions param is present'
       it_behaves_like 'only file_name is present'
       it_behaves_like 'only content is present'
+      it_behaves_like 'invalid params error response'
       it_behaves_like 'snippets spam check is performed' do
         before do
           subject

spec/support/shared_examples/services/snippets_shared_examples.rb

@@ -40,3 +40,20 @@ RSpec.shared_examples 'snippets spam check is performed' do
     end
   end
 end
+
+shared_examples 'invalid params error response' do
+  before do
+    allow_next_instance_of(described_class) do |service|
+      allow(service).to receive(:valid_params?).and_return false
+    end
+  end
+
+  it 'responds to errors appropriately' do
+    response = subject
+
+    aggregate_failures do
+      expect(response).to be_error
+      expect(response.http_status).to eq 422
+    end
+  end
+end