Ignore non-existing hooks pages

[srittau]

Add an "optional" flag to retrieve_data_gen() et al. that ignores 404
errors if set and generates an empty response. Set this flag when
implicitly requesting hooks with --all.

Closes: #176

[whwright]

IMO this should be handled through exception handling logic that is (kind of) in place already:

python-github-backup/github_backup/github_backup.py

Lines 1015 to 1016 in 18e78a4

	except SystemExit:
	log_info("Unable to read hooks, skipping")

Why not raise an more discrete exception so optional data like this can always be skipped over? This approach is very specific to "I used --all but hooks are optional"

[josegonzalez]

I don't see where a flag is set.

[josegonzalez]

The argument was never set so this is effectively dead code if merged. Closing, but feel free to revisit.