Improve error handling when attempting to bisect revision that doesn't exist

Attempts to bisect where the original revision doesn't exist (mistype, wrong branch, etc) will fail.

Traceback (most recent call last):
  File "/home/worker/.local/bin/bugmon-process", line 8, in <module>
    sys.exit(main())
  File "/home/worker/.local/lib/python3.8/site-packages/bugmon_tc/process/cli.py", line 33, in main
    results = processor.process()
  File "/home/worker/.local/lib/python3.8/site-packages/bugmon_tc/process/process.py", line 69, in process
    bugmon.process()
  File "/home/worker/.local/lib/python3.8/site-packages/bugmon/bugmon.py", line 397, in process
    self.confirm_open()
  File "/home/worker/.local/lib/python3.8/site-packages/bugmon/bugmon.py", line 258, in confirm_open
    self.bisect()
  File "/home/worker/.local/lib/python3.8/site-packages/bugmon/bugmon.py", line 208, in bisect
    bisector = Bisector(
  File "/home/worker/.local/lib/python3.8/site-packages/autobisect/bisect.py", line 168, in __init__
    self.end = Fetcher(
  File "/usr/local/lib/python3.8/dist-packages/fuzzfetch/fetch.py", line 805, in __init__
    requested = HgRevision(build, branch).pushdate
  File "/usr/local/lib/python3.8/dist-packages/fuzzfetch/fetch.py", line 132, in __init__
    self._data = get_url(
  File "/usr/local/lib/python3.8/dist-packages/fuzzfetch/fetch.py", line 107, in get_url
    raise FetcherException(exc) from None
fuzzfetch.fetch.FetcherException: 404 Client Error: Not Found for url: https://hg.mozilla.org/mozilla-central/json-rev/8be68465a851

Start in bugmon/bugmon.py at bisect(), then follow the Bisector and Fetcher calls shown in the traceback. Reproduce an attempt using the nonexistent revision 8be68465a851 and define completion as a handled error rather than an unhandled FetcherException traceback.