Fix spurious error message for prescribed mesh movement and multi-material.
Fix spurious error message for prescribed mesh movement and multi-material.

rjferrier/fluidity,rjferrier/fluidity,iakovos-panourgias/fluidity,rjferrier/fluidity,jjo31/ATHAM-Fluidity,jjo31/ATHAM-Fluidity,iakovos-panourgias/fluidity,jrper/fluidity,jjo31/ATHAM-Fluidity,iakovos-panourgias/fluidity,jrper/fluidity,rjferrier/fluidity,iakovos-panourgias/fluidity,jrper/fluidity,jjo31/ATHAM-Fluidity,iakovos-panourgias/fluidity,rjferrier/fluidity,jrper/fluidity,jrper/fluidity,jjo31/ATHAM-Fluidity