Improve error message for using protected keywords

Reported by @zhang_h:

An input like:

ring: Cyclotron, TYPE="RING", ...;

will generate an error:

Error>
Error> *** User error detected by function "OpalData::define()"
Error>     You cannot replace the object "RING".
Error>     You cannot replace the object "RING".

This is because ring is a protected keyword and can't be used as an object name. However, from the user perspective this seems not very clear.

The error message comes from OpalData.cpp:

        if (oldObject->isBuiltin()  ||  ! oldObject->canReplaceBy(newObject)) {
            throw OpalException("OpalData::define()",
                                "You cannot replace the object \"" + name + "\".");
        } else {

I propose to make a distinction between the two cases and change it to:

        if (oldObject->isBuiltin()) {
            throw OpalException("OpalData::define()",
                                "The keyword \"" + name + "\" is protected and can't be used to name an object.");
        else if (! oldObject->canReplaceBy(newObject)) {
            throw OpalException("OpalData::define()",
                                "You cannot replace the already defined object \"" + name + "\".");
        } // else not needed

Also it might be good not to have "name" in uppercase letters if the user writes it in lowercase, but I couldn't quickly find out where this happens.