Throw exceptions if the context isn't running during a call to begin()

It's easy to make this mistake--because the main thread is often the one making the call to begin, do not allow users to block on a call to `autowiring::parallel::begin` until after the context is started.

Fixes #688

autowiring/Parallel.h

@@ -131,6 +131,11 @@ class parallel {
   // Get an iterator to the begining of out queue of job results
   template<typename T>
   parallel_iterator<T> begin(void) {
+    if (!m_ctxt->IsRunning())
+      if (m_ctxt->IsShutdown())
+        throw std::runtime_error("Attempted to enumerate members of a context after the context was shut down");
+      else
+        throw std::runtime_error("Start the context before attempting to enumerate members of an autowiring::parallel collection");
     return{ *this, m_outstandingCount };
   }