Fix issues in loop control in TTreePlayer::Process

  In TTreePlayer::Process:
  - Make sure the same logic is used to condition the main loop and the Terminate/SlaveTerminate
    calls.
  - Fully rely on TSelector::EAbort for deciding if continue/abort processing and for the return
    value.
Use TSelector::EAbort in TSelectorDraw

Make TSelectorDraw to use TSelector::EAbort to signal problems during initialization,
i.e. during Begin.