Emit error codes when reporting diagnostics.

Addresses part of issue #506.

Before:
```
file.ts(1,6): ';' expected. 
file.ts(1,11): ';' expected.
```

After:
```
file.ts(1,6): Error TS1005: ';' expected. 
file.ts(1,11): Error TS1005: ';' expected.
```
why do we need this in the harness? this is more of a IO operation, like formatting just before writing.