Need a better error message for `yield` outside a generator

[Repro][1]

The following input

```js
yield;
```

produces a very cryptic error message:

```
input0:1: ERROR - Parse error. primary expression expected
yield;
^
```

It should probably be improved to something like "ERROR - yield only allowed inside a generator". Presumably this is as simple as just adding an additional case in Parser#parsePrimaryExpression right before the `default` to throw a different error for a `YIELD` token.

[1]: https://closure-compiler-debugger.appspot.com/#input0%3Dyield%253B%26input1%26conformanceConfig%26externs%26refasterjs-template%26includeDefaultExterns%3D1