[Sync] Don't crash for encryption errors

sync/util/cryptographer.cc

 // Copyright (c) 2012 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 "sync/util/cryptographer.h"
 
 #include <algorithm>
 
 #include "base/base64.h"
 #include "base/basictypes.h"
@@ skipping 109 matching lines @@
                             ::google::protobuf::MessageLite* message) const {
   DCHECK(message);
   std::string plaintext = DecryptToString(encrypted);
   return message->ParseFromString(plaintext);
 }
 
 std::string Cryptographer::DecryptToString(
     const sync_pb::EncryptedData& encrypted) const {
   NigoriMap::const_iterator it = nigoris_.find(encrypted.key_name());
   if (nigoris_.end() == it) {
-    NOTREACHED() << "Cannot decrypt message";
-    return std::string();  // Caller should have called CanDecrypt(encrypt).
+    // The key used to encrypt the blob is not part of the set of installed
+    // nigoris.
+    LOG(ERROR) << "Cannot decrypt message";
+    return std::string();
   }
 
   std::string plaintext;
   if (!it->second->Decrypt(encrypted.blob(), &plaintext)) {
     return std::string();
   }
 
   return plaintext;
 }
 
@@ skipping 232 matching lines @@
     NOTREACHED();
     return false;
   }
 
   if (!AddKeyImpl(nigori.Pass(), true))
     return false;
   return true;
 }
 
 }  // namespace syncer