Crash when a function with anonymous args is missing an argument

[sixolet]

In #2607 (comment) Guido points out that code like this crashes:

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.