Fix various issues with body inertial parsing and compiling.

- Add errors in body compiler for incorrect mixing of specifiers.
- Correctly process inertia orientation specifiers.

PiperOrigin-RevId: 676843326
Change-Id: I7e7eb22e25a6769754794f98fad5e0dcc0e5946f

src/xml/xml_native_reader.cc

@@ -3443,7 +3443,7 @@ void mjXReader::Body(XMLElement* section, mjsBody* body, mjsFrame* frame,
       bool alt = ReadAlternative(elem, body->ialt);
       bool full = ReadAttr(elem, "fullinertia", 6, body->fullinertia, text);
       if (alt && full) {
-        throw mjXError(elem, "fullinertia and orientation specifiers cannot be used together");
+        throw mjXError(elem, "fullinertia and inertial orientation cannot both be specified");
       }
     }