When named arguments introduce eta arguments, the full application
contains fvars for these eta arguments, so `MVarErrorKind.implicitArg`
needs to keep a local context for its error messages. This is because
the local context of the mvar associated to the `MVarErrorKind` is not
sufficient, since when an eta argument come after an implicit argument,
the implicit argument's mvar doesn't contain the eta argument's fvar in
its local context.
Closes#5475
