Rework console output:
- new log level "error"
- Replace nearly all calls to system.out and system.err to Main.(error|warn|info|debug)
- Remove some unnecessary debug output
- Some messages are modified (removal of "Info", "Warning", "Error" from the message itself -> notable i18n impact but limited to console error messages not seen by the majority of users, so that's ok)

git-svn-id: http://josm.openstreetmap.de/svn/trunk@6248 0c6e7542-c601-0410-84e7-c038aed88b3b