Improve error messages and reporting in GN

tools/gn/input_file_manager.cc

 // Copyright (c) 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.
 
 #include "tools/gn/input_file_manager.h"
 
 #include "base/bind.h"
 #include "base/stl_util.h"
 #include "tools/gn/filesystem_utils.h"
 #include "tools/gn/parser.h"
@@ skipping 28 matching lines @@
   // Read.
   base::FilePath primary_path = build_settings->GetFullPath(name);
   ScopedTrace load_trace(TraceItem::TRACE_FILE_LOAD, name.value());
   if (!file->Load(primary_path)) {
     if (!build_settings->secondary_source_path().empty()) {
       // Fall back to secondary source tree.
       base::FilePath secondary_path =
           build_settings->GetFullPathSecondary(name);
       if (!file->Load(secondary_path)) {
         *err = Err(origin, "Can't load input file.",
-                   "Unable to load either \n" +
-                   FilePathToUTF8(primary_path) + " or \n" +
+                   "Unable to load:\n  " +
+                   FilePathToUTF8(primary_path) + "\n"
+                   "I also checked in the secondary tree for:\n  " +
                    FilePathToUTF8(secondary_path));
         return false;
       }
     } else {
       *err = Err(origin,
                  "Unable to load \"" + FilePathToUTF8(primary_path) + "\".");
       return false;
     }
   }
   load_trace.Done();
@@ skipping 247 matching lines @@
 
   // Run pending invocations. Theoretically we could schedule each of these
   // separately to get some parallelism. But normally there will only be one
   // item in the list, so that's extra overhead and complexity for no gain.
   if (success) {
     for (size_t i = 0; i < callbacks.size(); i++)
       callbacks[i].Run(unowned_root);
   }
   return success;
 }