Update service to handle unexpected exceptions

This will ensure that now and in the future, PushOptionsHandlerService
will not cause the post_receive API endpoint from running other code if
something causes an unknown exception.

app/services/merge_requests/push_options_handler_service.rb

@@ -24,6 +24,9 @@ module MergeRequests
         execute_for_branch(branch)
       rescue Gitlab::Access::AccessDeniedError
         errors << 'User access was denied'
+      rescue StandardError => e
+        Gitlab::AppLogger.error(e)
+        errors << 'An unknown error occurred'
       end
 
       self

spec/services/merge_requests/push_options_handler_service_spec.rb

@@ -334,6 +334,32 @@ describe MergeRequests::PushOptionsHandlerService do
     end
   end
 
+  describe 'handling unexpected exceptions' do
+    let(:push_options) { { create: true } }
+    let(:changes) { new_branch_changes }
+    let(:exception) { StandardError.new('My standard error') }
+
+    def run_service_with_exception
+      allow_any_instance_of(
+        MergeRequests::BuildService
+      ).to receive(:execute).and_raise(exception)
+
+      service.execute
+    end
+
+    it 'records an error' do
+      run_service_with_exception
+
+      expect(service.errors).to eq(['An unknown error occurred'])
+    end
+
+    it 'writes to Gitlab::AppLogger' do
+      expect(Gitlab::AppLogger).to receive(:error).with(exception)
+
+      run_service_with_exception
+    end
+  end
+
   describe 'when target is not a valid branch name' do
     let(:push_options) { { create: true, target: 'my-branch' } }
     let(:changes) { new_branch_changes }