dart:io | Improve text of FileIOException on File.exists().

sdk/lib/io/file_impl.dart

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 part of dart.io;
 
 // Read the file in blocks of size 64k.
 const int _BLOCK_SIZE = 64 * 1024;
 
 
@@ skipping 210 matching lines @@
   // Constructor from Path for file.
   _File.fromPath(Path path) : this(path.toNativePath());
 
   Future<bool> exists() {
     _ensureFileService();
     List request = new List(2);
     request[0] = _EXISTS_REQUEST;
     request[1] = _path;
     return _fileService.call(request).then((response) {
       if (_isErrorResponse(response)) {
-        throw _exceptionFromResponse(response, "Cannot open file '$_path'");
+        throw _exceptionFromResponse(response,
+                                     "Cannot check existence of file '$_path'");
       }
       return response;
     });
   }
 
   external static _exists(String path);
 
   bool existsSync() {
     var result = _exists(_path);
     throwIfError(result, "Cannot check existence of file '$_path'");
@@ skipping 767 matching lines @@
           new FileIOException("File closed '$_path'"));
     });
     return completer.future;
   }
 
   final String _path;
   int _id;
 
   SendPort _fileService;
 }