Rewrite: Add proxy_intercept_errors to Spice endpoints.

When we get errors from the endpoint, instead of replying a blank page, it should reply the function instead with no parameters, e.g., `ddg_spice_dictionary_definition();`. The benefit of doing that is that we know for sure that the Spice failed, and we can do something about it (we know that the Spice failed because it should return `Spice.failed('...')` when the parameters are not valid).