Fix spurious failures to fetch tags for some projects
Fix spurious failures to fetch tags for some projects

Fixes an issue introduced in #471:

Occasionally, a project that actually had tags might be reported not
to, because an interrupted attempt to load the versions for it would
populate our cache with an empty array.

abbeycode/Carthage,erichoracek/Carthage,abbeycode/Carthage,abbeycode/Carthage,erichoracek/Carthage,erichoracek/Carthage,erichoracek/Carthage,abbeycode/Carthage