Provide more informative error message

Now when there's `applyNext()` in template body not wrapped in `function() {}` error message and stack trace are far from useful:

```
BEMHTML error: Cannot read property 'exec' of null
TypeError: Cannot read property 'exec' of null
```

Live example: https://goo.gl/Oiuy8q

Reproduce the live example with applyNext() directly in a template body and inspect the error-handling path that produces the current BEMHTML error and stack trace. Done means the invalid usage reports a specific, informative error with a useful stack trace.

Written by the indexing model from the issue text.