Fail when saving page as MHTML provides information about the cause.

content/common/frame_messages.h

 // Copyright 2013 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.
 
 // IPC messages for interacting with frames.
 // Multiply-included message file, hence no include guard.
 
 #include <stddef.h>
 #include <stdint.h>
 
 #include <map>
 #include <set>
 #include <string>
 #include <vector>
 
 #include "build/build_config.h"
 #include "cc/surfaces/surface_id.h"
 #include "cc/surfaces/surface_sequence.h"
 #include "content/common/content_export.h"
 #include "content/common/content_param_traits.h"
 #include "content/common/content_security_policy_header.h"
+#include "content/common/download/mhtml_save_status.h"
 #include "content/common/frame_message_enums.h"
 #include "content/common/frame_owner_properties.h"
 #include "content/common/frame_replication_state.h"
 #include "content/common/navigation_gesture.h"
 #include "content/common/navigation_params.h"
 #include "content/common/savable_subframe.h"
 #include "content/public/common/color_suggestion.h"
 #include "content/public/common/common_param_traits.h"
 #include "content/public/common/console_message_level.h"
 #include "content/public/common/context_menu_params.h"
@@ skipping 1423 matching lines @@
 
 // Response to FrameMsg_GetSerializedHtmlWithLocalLinks.
 IPC_MESSAGE_ROUTED2(FrameHostMsg_SerializedHtmlWithLocalLinksResponse,
                     std::string /* data buffer */,
                     bool /* end of data? */)
 
 // Response to FrameMsg_SerializeAsMHTML.
 IPC_MESSAGE_ROUTED4(
     FrameHostMsg_SerializeAsMHTMLResponse,
     int /* job_id (used to match responses to requests) */,
-    bool /* true if success, false if error */,
+    content::MhtmlSaveStatus /* final success/failure status */,
     std::set<std::string> /* digests of uris of serialized resources */,
     base::TimeDelta /* how much time of the main render thread was used */)
 
 // Sent when the renderer updates hint for importance of a tab.
 IPC_MESSAGE_ROUTED1(FrameHostMsg_UpdatePageImportanceSignals,
                     content::PageImportanceSignals)
 
 // This message is sent from a RenderFrameProxy when sequential focus
 // navigation needs to advance into its actual frame.  |source_routing_id|
 // identifies the frame that issued this request.  This is used when pressing
@@ skipping 57 matching lines @@
 // nearest find result in the sending frame.
 IPC_MESSAGE_ROUTED2(FrameHostMsg_GetNearestFindResult_Reply,
                     int /* nfr_request_id */,
                     float /* distance */)
 #endif
 
 // Adding a new message? Stick to the sort order above: first platform
 // independent FrameMsg, then ifdefs for platform specific FrameMsg, then
 // platform independent FrameHostMsg, then ifdefs for platform specific
 // FrameHostMsg.