Error handling for require-tree

fixes #2507

- ideally we would create an apps directory if one isn't present, but we can deal with that in later versions
Merge pull request #2508 from ErisDS/issue-2507

Error handling for require-tree
Cache invalidation for tag pages
Updating Ghost-UI to 0.1.3
Version bump for 0.4.2
Fixing showdown link to work without git