refs: TryGhost/Toolbox#147
* Replaces all references to isIgnitionError with isGhostError
* Switches use of GhostError to InternalServerError - as GhostError is no longer public
There are places where InternalServerError is not the valid error, and new errors should be added to the @tryghost/errors package to ensure that we can use semantically correct errors in those cases.
- The helper registration code is "framework" code and very specific
- At the moment the "theme engine" is full of lots of disparate theme related stuff
- I'm trying to make the frontend framework code clearer and also expand it to make it more useful
- The helper system now also exposes 3 methods allowing you to register a directory, a helper or an alias
- I've updated the codebase to use these both for our core helpers and for "apps"
