Expired GPG key for PQM user not handled gracefully

If the GPG key for the user running PQM expires, all merge requests will fail with the following traceback:

gpg: no default secret key: unusable secret key
gpg: [stdin]: clearsign failed: unusable secret key
Traceback (most recent call last):
  File "/home/pqm/pqm/bin/pqm", line 435, in ?
    run(pqm_subdir, run_mode, queuedir, logger, logdir, mail_reply, mail_server, from_address, fromaddr, print_report)
  File "/home/pqm/pqm/bin/pqm", line 201, in run
    do_run_mode(queuedir, logger, logdir, mail_reply, mail_server, from_address, fromaddr, print_report)
  File "/home/pqm/pqm/bin/pqm", line 91, in do_run_mode
    run_one_script(logger, script, logdir, goodscripts, badscripts, mail_reply, mail_server, from_address, fromaddr)
  File "/home/pqm/pqm/bin/pqm", line 125, in run_one_script
    (successes, unrecognized, output) = script.run()
  File "/home/pqm/pqm/pqm/__init__.py", line 221, in run
    command_result = command.run()
  File "/home/pqm/pqm/pqm/__init__.py", line 787, in run
    line='merge %s %s' % (self.from_branch, self.to_branch))
  File "/home/pqm/pqm/pqm/__init__.py", line 547, in do_merge
    self.get_vcs().commit(sender, dir, self.commitmsg, to_repo_revision, config)
  File "/home/pqm/pqm/pqm/__init__.py", line 1395, in commit
    tree.commit(summary, verbose=False)
  File "<string>", line 4, in commit_write_locked
  File "/home/pqm/source/bzr.dev/bzrlib/workingtree_4.py", line 246, in commit
    result = WorkingTree3.commit(self, message, revprops, *args, **kwargs)
  File "<string>", line 4, in commit_write_locked
  File "/home/pqm/source/bzr.dev/bzrlib/mutabletree.py", line 187, in commit
    revprops=revprops, *args, **kwargs)
  File "/home/pqm/source/bzr.dev/bzrlib/commit.py", line 373, in commit
    self.rev_id = self.builder.commit(self.message)
  File "/home/pqm/source/bzr.dev/bzrlib/repository.py", line 138, in commit
    self.new_inventory, self._config)
  File "<string>", line 4, in add_revision_write_locked
  File "/home/pqm/source/bzr.dev/bzrlib/repository.py", line 547, in add_revision
    self.store_revision_signature(
  File "<string>", line 4, in store_revision_signature_write_locked
  File "/home/pqm/source/bzr.dev/bzrlib/repository.py", line 1107, in store_revision_signature
    signature = gpg_strategy.sign(plaintext)
  File "/home/pqm/source/bzr.dev/bzrlib/gpg.py", line 98, in sign
    raise errors.SigningFailed(self._command_line())
bzrlib.errors.SigningFailed: Failed to gpg sign data with command "['gpg', '--clearsign']"

The failure message sent to the submitter will be:

Failed to gpg sign data with command "['gpg', '--clearsign']"