[dist] Improve logging

Give more of a clue when unable to reach the rep server.
[webui] proper error handling for changing review state
[webui] proper error handling when superseding a request.
[backend] support definition of predecessor product name

bnc#883206
[backend] fix forwarding of full tree cpio requests

Only needed for "transitive interconnect" setups, which are currently
not really supported anyway. Still...
[webui][api] Refactor attributes