buffer: Fail early and print diagnostic message
buffer: Fail early and print diagnostic message

When /tmp/mist/ was created before as a different user, and MistBuffer was
started, it would print "End of input file - buffer shutting down". This is very
confusing, therefore fail earlier and print a more useful message even if DEBUG
is disabled.

DDVTECH/mistserver,rbouqueau/mistserver,DDVTECH/mistserver,rbouqueau/mistserver,DDVTECH/mistserver,DDVTECH/mistserver,rbouqueau/mistserver,DDVTECH/mistserver,rbouqueau/mistserver,rbouqueau/mistserver