Handle message properties in a uniform style

As described in DERBY-6828, there are two styles used for handling
the translatable error message property files:

MessageService localization is used for the core Derby engine property
files, while LocalizedResource is used for the tools messages and
drda messages.

It seems as though it would be preferable to have a single approach
here. Potentially we could remove some duplicated code, but more
importantly it would just be nice to have the approach be consistent
so that Derby developers don't have to be aware of the two different
styles and be confused by them.