Create :Errors command to populate the quickfix list with Accio errors for the current buffer on demand

[pgdouyon]

• The quickfix list, while more consistent and easier to access than the
  location list, can only hold one list at a time, meaning Accio errors will
  probably be overwritten by future quickfix commands. It would be nice to have
  an easy way to recall that list of errors for each buffer.
• The quickfix list will even be overridden by other Accio commands