fix #8822 - Don't try to fix empty errors

git-svn-id: http://josm.openstreetmap.de/svn/trunk@6030 0c6e7542-c601-0410-84e7-c038aed88b3b
see #8809: [patch by brycenesbitt, reworked] detailed documentation for "no suitable download task is available" error (Ctrl-L)

git-svn-id: http://josm.openstreetmap.de/svn/trunk@6031 0c6e7542-c601-0410-84e7-c038aed88b3b