Author: Petr Škoda
MDL-34441 improve error handling when migrating databases
http://git.moodle.org/gw?p=moodle.git;a=commit;h=0dd05d20b937edd7fd0beb9b30f051fdfa927e64
http://github.com/moodle/moodle/commit/0dd05d20b937edd7fd0beb9b30f051fdfa927e64
+ 2.3 en [targetdatabasenotempty,tool_dbtransfer]