Automate checking existing fallbacks

I use a manual process to check existing fallbacks. It would be great if the updateFallbackDirs.py script would automatically read src/or/fallback_dirs.inc, and check each fallback for errors.

For details, see: https://trac.torproject.org/projects/tor/wiki/doc/UpdatingFallbackDirectoryMirrors

I think it can go in 0.3.0

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