@greg, this is a general problem with CI and release. API clients shouldn't need to detect defective API modules on WMF servers, or any MediaWiki installation for that matter.
In addition to T100775: fancycaptchareload paraminfo failure on test wikis (May) , T108317: paraminfo for flow submodule view-topic-history fails is another case which has occurred now (August). Those are the two that the Pywikibot team is aware of, as they have broken Pywikibot tests. Have there been other cases in 2015?
IMO WMF needs unit tests to verify that changes dont break paraminfo. T100839: Run test suites for all captcha types in ConfirmEdit has gone nowhere.
Also, integration tests running on beta should detect broken modules. T100903: Run pywikibot test suite regularly on beta cluster as part of MediaWiki/Wikimedia CI would provide this. Fixing T28680: Add a way to allow action=paraminfo to dump results for all modules would also make testing this much easier, as one API call would test every module.
Finally, IMO MediaWiki paraminfo API should also better handle when a broken module is loaded to the live wiki, instead of 503 being considered an appropriate API response.