Adding extra error message in unitMultiplierFactorDifference to reflect current behaviour
Adding extra error message in unitMultiplierFactorDifference to reflect current behaviour

nickerso/libcellml,hsorby/libcellml,cellml/libcellml,nickerso/libcellml,MichaelClerx/libcellml,MichaelClerx/libcellml,hsorby/libcellml,nickerso/libcellml,cellml/libcellml,hsorby/libcellml,cellml/libcellml,MichaelClerx/libcellml,hsorby/libcellml,cellml/libcellml,nickerso/libcellml