add self.name to configure_proxy error message

For example, writing a plugin mastakenly like:

```
# out_stdout.rb
config_param :foo, :default => true
config_param :foo, :default => false
```

Before: error message shows as:

```
2014-11-25 18:44:14 +0900 [error]: unexpected error error="default value specified twice for foo"
```

After: 

```
2014-11-25 18:44:14 +0900 [error]: unexpected error error="Fluent::StdoutOutput: default value specified twice for foo"
```

We ~~can not~~ can find which plugin is doing bad easily now. 

Just same thing for others.