[BUG] Poor error message when a function signature uses a keyword that was removed/hasn't been added yet

[nmsmith]

Bug description

In argument lists, if the compiler encounters two identifiers in succession, for example fn foo(a b), the compiler currently complains that it "expected )". This is not very informative. Most of the time, I'd wager the reason the programmer has written two identifiers is that they expected the first identifier to be a keyword. The Mojo compiler should account for this, and perhaps say something like:

Did not expect to encounter identifier b after identifier a. Note: a is not a keyword.

This improvement to error messages seems especially important for today's Mojo, given that in each new release of Mojo, new keywords are being introduced, and old keywords are being removed.

Steps to reproduce

Mojo 24.5 doesn't have the out keyword, so running the following program on 24.5 produces a mysterious error:

struct Foo:
    var x: Int
    fn __init__(out self, x: Int):
        self.x = x

error: expected ')' in argument list
    fn __init__(out self, x: Int):
                    ^

At least one Mojo learner has encountered this poor error message.

System information

No response