Rollup merge of #139024 - compiler-errors:tweak-default-value-err, r=lcnr

Make error message for missing fields with `..` and without `..` more consistent

When `..` is not present, we say "missing field `bar` in initializer", but when it is present we say "missing mandatory field `bar`". I don't see why the primary error message should change, b/c the root cause is the same.

Let's harmonize these error messages and instead use a label to explain that `..` is required b/c it's not defaulted.

r? estebank

Author: Zalathar