better error message for missing comma.

[oscardssmith]

When missing a , in a list or function args, the error message currently says "expected )" when the real problem is the missing comma. Should the error message be updated to add that as a possibility? (examples where this comes up are +(1, 2 3) and [1, 2 3])

[c42f]

We should be able to do better here 👍

We could probably emit "expected , or closing )" easily.

Distinguishing between possible recoveries and choosing the "best" one is a lot harder.