message: Fix error message in FileViewer in absence of file
  Fixes
diff: @@ -348,7 +348,7 @@ void FileViewer::SelectReference(const wxString& ref)
  const wxFileName filename = GetFilename(ref);
  if (!filename.IsOk())
  {
  - ShowError(wxString::Format(_("Error opening file %s!"), filename.GetFullPath()));
  + ShowError(wxString::Format(_("Error opening file %s!"), ref.BeforeLast(':')));
  m_openInEditor->Disable();
  return;
  }
  @@ -360,7 +360,7 @@ void FileViewer::SelectReference(const wxString& ref)
  !file.Open(filename.GetFullPath()) ||
  !file.ReadAll(&data, wxConvAuto()) )
  {
  - ShowError(wxString::Format(_("Error opening file %s!"), filename.GetFullPath()));
  + ShowError(wxString::Format(_("Error opening file %s!"), ref.BeforeLast(':')));
  m_openInEditor->Disable();
  return;
  }
  