Improve warnings and messages behavior

[avehtari]

• Go through all warnings and messages in loo code
• Make it easier to suppress warnings and messages with options
• Avoid printing "Warning: " when actually using message()
• Add here a list of all issues which refer to warnings and messages
  • Rely on posterior for pareto smooth tails #290
• Check which warnings and messages come from posterior, see also Warning levels posterior#309