bzr lp-propose fails if there's an existing merged proposal

[Martin Pool]

If there's an existing closed proposal for a branch, lp-propose fails, but it should instead create a new one:

-Derror lp-propose
bzr: ERROR: bzrlib.errors.BzrCommandError: There is already a branch merge proposal: https://code.edge.launchpad.net/~mbp/bzr/doc/+merge/25333

Traceback (most recent call last):
  File "/home/mbp/bzr/trunk/bzrlib/commands.py", line 911, in exception_to_return_code
    return the_callable(*args, **kwargs)
  File "/home/mbp/bzr/trunk/bzrlib/commands.py", line 1109, in run_bzr
    ret = run(*run_argv)
  File "/home/mbp/bzr/trunk/bzrlib/commands.py", line 689, in run_argv_aliases
    return self.run(**all_cmd_args)
  File "/home/mbp/bzr/trunk/bzrlib/commands.py", line 704, in run
    return self._operation.run_simple(*args, **kwargs)
  File "/home/mbp/bzr/trunk/bzrlib/cleanup.py", line 122, in run_simple
    self.cleanups, self.func, *args, **kwargs)
  File "/home/mbp/bzr/trunk/bzrlib/cleanup.py", line 156, in _do_with_cleanups
    result = func(*args, **kwargs)
  File "/home/mbp/bzr/trunk/bzrlib/plugins/launchpad/__init__.py", line 344, in run
    proposer.check_proposal()
  File "/home/mbp/bzr/trunk/bzrlib/plugins/launchpad/lp_propose.py", line 156, in check_proposal
    canonical_url(mp))
BzrCommandError: There is already a branch merge proposal: https://code.edge.launchpad.net/~mbp/bzr/doc/+merge/25333