rdtech-um: tone down and extend diagnostics messages

rdtech-um: tone down and extend diagnostics messages

Reflect tested conditions in diagnostics messages, and tell more
conditions apart where possible. Take the user's perspective in these
messages.

Address programming style while we are here: Prefer read_u16be() over
RB16() for robustness. Use intermediate variables to avoid repeated
converter calls. Transparently advance over the received data length
(the read size and the separated increment had no perceivable relation
in the source code).