message: Much better FileViewer UI when sources aren't available
  If we can't show source code files, show a nice error instead, and
  include an explanation and all references in it. This provides at least
  some information and also works for non-file references.
diff: @@ -60,7 +60,8 @@ private:
  wxFileName GetFilename(wxString ref) const;
  void SelectReference(const wxString& ref);
  - void ShowError(const wxString& msg);
  + void ShowHTMLContent(const wxString& markup);
  + void ShowError(const char *icon, const wxString& msg, const wxString& description = "", const wxString& references = "");
  private:
  wxString m_basePath;
  @@ -70,7 +71,6 @@ private:
  wxStaticText *m_description;
  wxButton *m_openInEditor;
  wxWebView *m_content;
  - wxStaticText *m_error;
  wxSizer *m_topBarSizer;
  void OnChoice(wxCommandEvent &event);
  