Consistently underline error spans with '^'

[ratmice]

This makes error messages consistently underline with the '^' character,

As discussed in #578, using different underline characters was intended for a more footnote style,
where we would avoid emitting duplicate source lines, by coalescing spans and use multiple underlines
on a single source line, then emitting error text as footnotes. But I never actually implemented that,
and it seems like it would probably be more difficult to read/follow.

[ltratt]

Thanks!