Fix [RTI]: rescue NameError and NoMethodError on invalid type field value

When a `type` field is set in an Acfs-Resource, Acfs mimics Rails' STI by trying to instantiate a resource with the name of `type.camelize.constantize`. This fails, if the type string does not represent a valid subclass of the resource.
For better traceability, a `ResourceTypeError` indicating the type mismatch gets raised.
Merge pull request #18 from nicolas-fricke/fix/better_error_for_invalid_resource_type

[RTI] Fix: rescue invalid type field errors