Message of not able to associate motion to a model can be improved

[aymanhab]

Steps to reproduce

Load model, and a motion file from a different model (e.g. arm26.osim, .mot file from leg39 model)

Expected result

Clear message regarding why the motion file doesn't correspond to model

Actual result

Message: Could not associate motion to the current model. (include screen grabs where appropriate)

Environment and GUI version

[aymanhab]

Make the message more clear (e.g. Could not associate motion to the current model because the file doesn't contain coordinate values)