Better error messages when loading plugins

git-svn-id: http://josm.openstreetmap.de/svn/trunk@5987 0c6e7542-c601-0410-84e7-c038aed88b3b
fix EDT violations observed while merging nodes

git-svn-id: http://josm.openstreetmap.de/svn/trunk@5988 0c6e7542-c601-0410-84e7-c038aed88b3b
fix #8760 - When merging nodes, keep the oldest used one, not just only the oldest one.

git-svn-id: http://josm.openstreetmap.de/svn/trunk@5989 0c6e7542-c601-0410-84e7-c038aed88b3b