Converted in-source 'TODO' comments to standard form.
Quote hook event, as it may contain space.
Merge pull request #412 from matthewrmshin/quote-hook-event

Quote hook event, as it may contain space.
Merge branch 'master' of github.com:cylc/cylc
Hot fix: print a validation warning to stderr.

Closes #413
Minor doc change only; closes #410.