Why:
Passing an empty string as an endpoint to Client when instantiating a
new client might seem like something that should never happen but I
managed to trigger it while parsing some input files to register feeds
in bulk.
What:
In the execute() function, check early if the endpoint is "" and then
return immediately nil and a new error, named ErrEmptyEndpoint with a
descriptive string
Try to parse the response body from the server when an HTTP 500 is
returned (i.e. http.StatusInternalServerError) as it might contain
useful information. If successful, create a new error and append that
information to the returned error message. Otherwise just maintain the
same behavior
