Clarify JointCannotJoinTheSamePhysicalFrame error message.
Clarify JointCannotJoinTheSamePhysicalFrame error message.

opensim-org/opensim-core,opensim-org/opensim-core,opensim-org/opensim-core,opensim-org/opensim-core,opensim-org/opensim-core,opensim-org/opensim-core,opensim-org/opensim-core