'--watch' output displays diagnostic category/code

```
$ node built/local/tsc.js --watch src/services/services.ts src/services/shims.ts --out built/local/tsc2.js
message TS6042: Compilation complete. Watching for file changes.
message TS6032: File change detected. Compiling...
message TS6042: Compilation complete. Watching for file changes.
```

I'm pretty sure we don't want to print out `message TS1337:` each time.
