triggers use JSON and yield errors when a trigger attempts to pass a non-unicode name through JSON

On version 4.8.0-rc1, I get this error message consistently on first start from one of my watches:

```
2017-03-09T20:55:17,531: [trigger somewhere-files /home/somewhere] input_json: sending json object to stm
2017-03-09T20:55:17,531: [trigger somewhere-files /home/somewhere] input_json: failed to write json data to stream: Socket operation on non-socket
2017-03-09T20:55:17,531: [trigger somewhere-files /home/somewhere] trigger /home/somewhere:somewhere-files Socket operation on non-socket
```

```
# watchman trigger-list /home/somewhere/
{
"version": "4.8.0",
"triggers": [
{
"append_files": false,
"name": "somewhere-files",
"relative_root": "files/",
"empty_on_fresh_instance": true,
"stdin": [
"name",
"exists"
],
"command": [
"/usr/local/bin/xfer_notify",
"Somewhere"
],
"expression": [
"type",
"f"
],
"dedup_results": true
}
]
}
```

The same problem happens on 4.7.0 stable, except there the system reports "input_json: failed to write json data to stream: Success", which is presumably just a bug in the logging.