Crash when a function with anonymous args is missing an argument

In https://github.com/python/mypy/pull/2607#issuecomment-299019741 Guido points out that code like this crashes:

```python
def foo(__b: int, x: int) -> int: pass
foo(x=2)
```

This is because I forgot to make sure error messages could handle anonymous arguments.  I'll have a fix for this in soon, and scan the rest of the error messages for similar bugs.
