[frontend] Add missing newline to parrot_debugger flags error message

parrot_debugger doesn't take -x or --xxxx flags.  The error message didn't
have a newline, and thus the prompt doesn't return properly.  This commit
fixes this issue.
[t/run] Add a test for correct flags error message from parrot_debugger
Update MANIFEST after addition of debugger_options.t
Merge pull request #961 from paultcochrane/ptc/debugger_errmsg_fix

Add newline to parrot_debugger error message