Log the rpc call name in the RPC errors message

Currently the rpc module logs the error description and target node in
rpc calls logging, as such:

  2009-01-21 00:50:01,456:  pid=1051/Thread-21 ERROR RPC error from node
    node1.example.com: Connection failed (111: Connection
    refused)

but this doesn't help to understand which call caused this (here it's an
offline node which should not be contacted at all).

This patch adds the logging of the call too, so cases like the above can
be debugged easier.

Reviewed-by: imsnah, ultrotter