GN: Make file/dir resolving return errors.

tools/gn/source_file.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/source_file.h"
 
 #include "base/logging.h"
 #include "build/build_config.h"
 #include "tools/gn/filesystem_utils.h"
 #include "tools/gn/source_dir.h"
 
 namespace {
 
 void AssertValueSourceFileString(const std::string& s) {
 #if defined(OS_WIN)
   DCHECK(s[0] == '/' ||
          (s.size() > 2 && s[0] != '/' && s[1] == ':' && IsSlash(s[2])));
 #else
   DCHECK(s[0] == '/');
 #endif
-  DCHECK(!EndsWithSlash(s));
+  DCHECK(!EndsWithSlash(s)) << s;
 }
 
 }  // namespace
 
 SourceFile::SourceFile() {
 }
 
 SourceFile::SourceFile(const base::StringPiece& p)
     : value_(p.data(), p.size()) {
   DCHECK(!value_.empty());
@@ skipping 42 matching lines @@
     }
     return base::FilePath(UTF8ToFilePath(converted));
   }
 
   converted.assign(&value_[2], value_.size() - 2);
   if (source_root.empty())
     return UTF8ToFilePath(converted).NormalizePathSeparatorsTo('/');
   return source_root.Append(UTF8ToFilePath(converted))
       .NormalizePathSeparatorsTo('/');
 }