Make error span matching more precise

[danvk]

There's currently some wiggle room to accommodate this pattern:

{
  longname = 12;
  // ~~~~~ Cannot find name 'longname'.
}

That's done by only requiring the end of the ~~~ to line up with the error. But unless it's at the beginning of the line (or the comment), the start of the ~~~ ought to line up, too.