[ci skip] Fixed #14: Clarify error when yielding a parallel request builder without requests

Did not implement the short-circuit to the converge method yet, as that would circumvent the controller check. Maybe later.

Tapeti.Flow/Default/FlowProvider.cs

@@ -290,6 +290,9 @@ namespace Tapeti.Flow.Default
 
             private IYieldPoint BuildYieldPoint(Delegate convergeMethod, bool convergeMethodSync)
             {
+                if (requests.Count == 0)
+                    throw new YieldPointException("At least one request must be added before yielding a parallel request");
+
                 if (convergeMethod?.Method == null)
                     throw new ArgumentNullException(nameof(convergeMethod));
 

docs/flow.rst

@@ -251,12 +251,10 @@ A few things to note:
 
 #) The response handlers do not return an IYieldPoint themselves, but void (for AddRequestSync) or Task (for AddRequest). Therefore they can not influence the flow. Instead the converge method as passed to Yield or YieldSync determines how the flow continues. It is called immediately after the last response handler.
 #) The converge method must be private, as it is not a valid message handler in itself.
-
+#) You must add at least one request.
 
 Note that you do not have to perform all the operations in one go. You can store the result of ``YieldWithParallelRequest`` and conditionally call ``AddRequest`` or ``AddRequestSync`` as many times as required.
 
-.. warning:: At the time of writing, you must add at least one request to the parallel request builder before yielding or your flow will halt. This will hopefully be fixed in the future.
-
 
 Persistent state
 ----------------

docs/indepth.rst

@@ -132,6 +132,7 @@ The routing key is determined by converting CamelCase to dot-separated lowercase
       public string Description { get; set; }
   }
 
+
 This behaviour is implemented using the IRoutingKeyStrategy interface. For more information about changing this, see `Overriding default behaviour`_