Group errors together with their final failure message.

This isn't strictly about issue #2, but it happens to make testing
the fixes for that issue much easier, since the in-progress fixes make
so many tests fail and one wants to see the groupings more clearly.

* check.sh
  (check_result): If fail, print an extra blank line afterwards.
  (everywhere): Add an extra blank line before error messages, so
    they're easier to parse visually and so they're more grouped
    with their associated final failure line.