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

[pyoor]

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