Raise exception if unsupported mix strategy name is specified

This patch fixes https://github.com/jubatus/jubatus/issues/600.

If we specified invalid MIX strategy name using `--mixer, -x` option, server fail to start like followings:

```
F0110 20:17:21.527549 17802 server_util.hpp:143] Dynamic exception type: jubatus::core::common::exception::runtime_error::what: unsupported mix type (no_mixer)
    #0 [jubatus::core::common::exception::error_at_file_*] = ../jubatus/server/framework/mixer/mixer_factory.cpp
    #0 [jubatus::core::common::exception::error_at_line_*] = 62
    #0 [jubatus::core::common::exception::error_at_func_*] = jubatus::server::framework::mixer::mixer* jubatus::server::framework::mixer::create_mixer(const jubatus::server::framework::server_argv&, const jubatus::util::lang::shared_ptr<jubatus::server::common::lock_service>&)
```