Added context information to Hamlib interface exception messages.

Enhanced TransceiverBase  error exception  type to allow  for language
translatable messages.

git-svn-id: svn+ssh://svn.code.sf.net/p/wsjt/wsjt/branches/wsjtx@4033 ab8295b8-cf94-4d9e-aec4-7959e3be5d79

PollingTransceiver.cpp

@@ -200,7 +200,7 @@ void PollingTransceiver::impl::handle_timeout ()
     }
   catch (...)
     {
-      message = "Unexpected rig error";
+      message = tr ("Unexpected rig error");
     }
   if (!message.isEmpty ())
     {