Fixes #85 (Slice input errors trigger Internal Server Error)

This pull request adds code to catch additional errors that could be thrown by the underlying RESTful interface and handle them correctly.  It also takes out the code that resulted in exceptions being returned directly in the body of the HTTP response, wrapping these exceptions up in a proper Rack::Response containing the error message and class in the JSON response.  Finally, this pull request modifies the Core::ImageService::Base class so that the errors returned when the file does not exist or it is not an ISO actually include the name of the file containing the image being added to Hanlon.  With these changes in place it should be much easier for users to determine why their request failed.