Better redirection handling.
* bzrlib/tests/test_script.py:
(TestRedirections): Explicit tests for redirections including the
cases used by Maritza Mendez in the #425510 and $426410 reports.
* bzrlib/tests/script.py:
(_scan_redirection_options.redirected_file_name): Factor out
redirection file name handling.
(_scan_redirection_options): Handle the case where the redirection
is specified with 2 arguments instead of one.
(ScriptRunner.run_command): Tweak error message.
(ScriptRunner.do_cat, ScriptRunner.do_echo): Catch redirection
errors.