improve error message when ) is missing on prev line

when closing ) is missing on a prev line error message `got identifier, want ','` pops up. since ytt shows error related line it's a bit confusing that ) is missing on the prev line.

- https://get-ytt.io/#gist:https://gist.github.com/cppforlife/c34961a5e9a5aa31a16d6d678ed28f96
- https://get-ytt.io/#gist:https://gist.github.com/cppforlife/8c7088cab2b7b64752fbc4481cac5a17

should we add some hint to indicate this similar to this: https://github.com/k14s/ytt/blob/582ad9ca849559e5d75b680b1831105059a0e953/pkg/template/error_hints_test.go

Reproduce the two linked examples to confirm the confusing `got identifier, want ','` error. Start with pkg/template/error_hints_test.go and the existing error-hint cases; done means the missing `)` is indicated clearly when the reported error line is the following line, with tests covering the examples.

Written by the indexing model from the issue text.