Remove a variable "msg" in THROW_HATOHOL_EXCEPTION()

The variable may shadow a parameter of THROW_HATOHOL_EXCEPTION().
This issue is notified by clang:

  clang: warning: argument unused during compilation: '-pthread'
  DBAgentSQLite3.cc:291:22: warning: variable 'msg' is uninitialized when used
  within its own initialization [-Wuninitialized]
  dbPath.c_str(), msg.c_str());
  ~~~~~~~~~~~~~~~~^~~~~~~~~~~~