Whitespace/indentation fix
Refactor to Node-style errback for applet init

With at least three different causes for error at applet initialization times, each of which are discovered asynchronously, this pattern makes a lot of sense.

IS [#49520689]
getTestAppletHTML/Codebase: not Vernier specific

TODO: getCodebase is specific to "Importer" experiment, whoops!
Fix actualRoot detection for new interactive home
Fix getCodebase because we can now use actualRoot