Backend: Print error messages to stderr.
Backend: Print error messages to stderr.

This also makes all user-generated errors (using the 'b'oom command) go
to stderr. Nobody objected (in fact, nobody said anything at all...).

Issue #104.

SWP-Ubau-SoSe2014-Haskell/SWPSoSe14,SWP-Ubau-SoSe2014-Haskell/SWPSoSe14