Remove unexpected's entries from the stack traces of the thrown error objects

This fix makes sure that the top stack frame is the place that calls `expect`, which makes the output easier to parse.

Previously you'd get something like:

```
Error: expected foo to be bar
      at Assertion.throwStandardError (/path/to/unexpected/lib/unexpected.js:41:15)
      at Assertion.assert (/path/to/unexpected/lib/unexpected.js:52:18)
      at Assertion.<anonymous> (/path/to/unexpected/lib/unexpected.js:258:18)
      at expect (/path/to/unexpected/lib/unexpected.js:67:21)
      at theActualTestSuite (/path/to/testSuite:22:13)
```

With this patch it'll be:

```
Error: expected foo to be bar
      at theActualTestSuite (/path/to/testSuite:22:13)
```