use tr in scanner plugin manager

Change-Id: I074341cdff0e3f50b9036c3de82fb8c1dbcbb62a
Reviewed-by: Christian Kandeler <christian.kandeler@digia.com>
show QLibrary::errorString if plugin loading fails

Change-Id: I8ec1073f8057e90a4a560cf642c2c83ab40db44a
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>
Reviewed-by: Christian Kandeler <christian.kandeler@digia.com>