Added a diagnostic user message when enumerating printers fails. Also tweaked...

printing/backend/print_backend.h

 // Copyright (c) 2010 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef PRINTING_BACKEND_PRINT_BACKEND_H_
 #define PRINTING_BACKEND_PRINT_BACKEND_H_
 #pragma once
 
 #include <map>
 #include <string>
@@ skipping 30 matching lines @@
 // obtain available print backend.
 // Please note, that PrintBackend is not platform specific, but rather
 // print system specific. For example, CUPS is available on both Linux and Mac,
 // but not available on ChromeOS, etc. This design allows us to add more
 // functionality on some platforms, while reusing core (CUPS) functions.
 class PrintBackend : public base::RefCountedThreadSafe<PrintBackend> {
  public:
   virtual ~PrintBackend();
 
   // Enumerates the list of installed local and network printers.
-  virtual void EnumeratePrinters(PrinterList* printer_list) = 0;
+  virtual bool EnumeratePrinters(PrinterList* printer_list) = 0;
 
   // Gets the capabilities and defaults for a specific printer.
   virtual bool GetPrinterCapsAndDefaults(
       const std::string& printer_name,
       PrinterCapsAndDefaults* printer_info) = 0;
 
   // Returns true if printer_name points to a valid printer.
   virtual bool IsValidPrinter(const std::string& printer_name) = 0;
 
   // Allocate a print backend. If |print_backend_settings| is NULL, default
   // settings will be used.
   // Return NULL if no print backend available.
   static scoped_refptr<PrintBackend> CreateInstance(
       const DictionaryValue* print_backend_settings);
 };
 
 }  // namespace printing
 
 #endif  // PRINTING_BACKEND_PRINT_BACKEND_H_