Define an AudacityException subclass for unready on-demand data

src/blockfile/NotYetAvailableException.cpp

@@ -0,0 +1,28 @@
+//
+//  NotYetAvailableException.cpp
+//  
+//
+//  Created by Paul Licameli on 12/25/16.
+//
+//
+
+#include "../Audacity.h"
+#include "NotYetAvailableException.h"
+
+NotYetAvailableException::~NotYetAvailableException()
+{
+}
+
+std::unique_ptr< AudacityException > NotYetAvailableException::Move()
+{
+   return std::unique_ptr< AudacityException >
+   { safenew NotYetAvailableException{ std::move( *this ) } };
+}
+
+wxString NotYetAvailableException::ErrorMessage() const
+{
+   return wxString::Format(
+      _("This operation cannot be done until importation of %s completes."),
+      mFileName.GetFullName()
+   );
+}