New error format

This is the result of a deliberate change we made, which was to convert multi-line spans into a single ^ at the start. This was made to lighten the display of multi-line spans:

  • today we just display the lines with no carrot at all; that seemed bad
  • before we were displaying ^^^^^ all the way until the end of the line, then some ..., and then ^^^ at the end; this was a bit much.

We could tweak this behavior. However, I think that the assumption that @jntrnr and I made was that by and large multi-line spans are a "bug" in the sense that we ought to try and supply tighter spans. For example, to highlight a method, we should just give the span of the method name, not the entire method definition.