refactor: factor out formatted error stream output.

* lib/specl/shell.lua (reformat_err): Renamed from this...
(reformat_errout): ...to this. Adjust all callers.
(nonempty_errout): New function...
(process_errout): ...factored out of here.  Simplify accordingly.
(matchers.exit.format_actual, Matchers.succeed.format_actual)
(matchers.succeed_while_containing.format_actual)
(matchers.succeed_with.format_actual)
(matchers.succeed_while_matching.format_actual)
(matchers.fail.format_actual): Simplify with nonempty_errout ().

Signed-off-by: Gary V. Vaughan <gary@gnu.org>