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()
+   );
+}

src/blockfile/NotYetAvailableException.h

@@ -0,0 +1,34 @@
+//
+//  NotYetAvailableException.h
+//  
+//
+//  Created by Paul Licameli on 12/25/16.
+//
+//
+
+#ifndef __AUDACITY_NOT_YET_AVAILABLE_EXCEPTION__
+#define __AUDACITY_NOT_YET_AVAILABLE_EXCEPTION__
+
+#include "../FileException.h"
+#include <wx/filename.h>
+
+// This exception can be thrown when attempting read of on-demand block files
+// that have not yet completed loading.
+class NotYetAvailableException final : public FileException
+{
+public:
+   NotYetAvailableException( const wxFileName &fileName )
+      : FileException{ Cause::Read, fileName } {}
+   NotYetAvailableException(NotYetAvailableException &&that)
+      : FileException( std::move( that ) ) {}
+   ~NotYetAvailableException();
+
+protected:
+   std::unique_ptr< AudacityException > Move() override;
+   wxString ErrorMessage() const override;
+
+private:
+   wxFileName mFileName;
+};
+
+#endif