- Place "Error:" inside wrappers

  At least for the sake of the testsuite, the "Error:" prefix must be
  inside the bracket BEGIN...END that is added to the Cabal output
  during testing.

- Heuristic to not put "Error:" when already present

  This is because backpack outputs errors in an aligned way (Doc) where
  "Error:" is the introducing line.  Removing this breaks the layout.

- Remove manual "Error:" prefix where `die` adds it automatically.

- Update golden values.