Improve error messages and reporting in GN

tools/gn/input_file_manager.cc

Index: tools/gn/input_file_manager.cc
diff --git a/tools/gn/input_file_manager.cc b/tools/gn/input_file_manager.cc
index 097474ae061c0b99953accf3341e668cbba9bbe7..eb255080a9da68445cec8b3d03e4c612a7b6ae3a 100644
--- a/tools/gn/input_file_manager.cc
+++ b/tools/gn/input_file_manager.cc
@@ -46,8 +46,9 @@ bool DoLoadFile(const LocationRange& origin,
           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;
       }