Fixes#356
- Adds new generic methods for handling errors to errorHandling.js
- Initialises generic methods as middleware
- Created error.hbs view in admin
- Error handler searches for error.hbs view file in user theme folder
and renders it if available, otherwise lets the error fall through
to express.
- We *could* change the final behaviour to render a default ghost
template should the user template be missing
- Because it currently isn't possible to require(ghost) in errorHandling.js,
it was necessary to duplicate some aspects of the ghost path init code
inside errorhandling.js. This should be cleaned up and moved back
into ghost.js when possible.
