print out returned content from http errors

When a http server returns an error (400, 500 or whatever status),
it can also return a message explaining why this happens. At the
moment this message is just eaten and not printed out, which makes
communication problems with web services like PHEDEx really
hard to debug. Add a printout.
Merge pull request #4822 from hufnagel/http-debug

print out returned content from http errors