The hard-coded PlanExporter help message should be internationalized so that it can be translated

The help message for PlanExporter is hard-coded in PlanExporter.printHelp(). In contrast, other Derby tools use the message lookup logic for their help messages. This means that for those other tools, on supported locales, users see translated versions of the help messages. For PlanExporter, the help message always appears in English. PlanExporter should use the message lookup logic like the other tools do.