Fixes #158 (Unhandled Exceptions/Errors in RESTful API)

The changes in this pull request ensure that the errors/exceptions that can be raised from within the Hanlon RESTful API (and from within the methods invoked by that API) are all caught and returned as appropriate HTTP responses. This pull request also aggregates the `rescue_from` blocks by HTTP status code (so that all exceptions that return a given HTTP status code are handled by the same block) and adds a block to catch any `ProjectHanlon::Error::Slice::InternalError` errors thrown from within the code explicitly (rather than relying on the block that handles the `:all` errors to catch them implicitly).