- only start working on/with banlist data, if reading in the banlist from
  disk didn't fail
- as CNode::setBannedIsDirty is false (default) when reading fails, we
  don't need to explicitly set it to false to prevent writing
  banlist.dat in that case either

- to match the peers.dat handling also supply a debug.log entry for how
  many entries were loaded from banlist.dat and how long it took
- add a GUI init message for loading the banlist (same as with peers.dat)

- move the same message for peers.dat upwards in the code, to be able to
  reuse the timing variable nStart and also just log, if our read from
  peers.dat didn't fail

- allows CNode::SweepBanned() to run, even if !CNode::BannedSetIsDirty(),
  because if nBanUntil is over we want the ban to be disabled for these
  nodes