Block matfaust.Faust by sparse matrix multiplication and display error msg.

pyfaust.Faust's already blocking Faust-sparse mul.

wrapper/matlab/+matfaust/@Faust/Faust.m

@@ -294,7 +294,9 @@ classdef Faust
 			if (trans ~= 1) && (trans ~= 0)
 				error('invalid argument trans, must be equal to 0 or 1');
 			end
-			if(isa(A,'matfaust.Faust'))
+			if(issparse(A))
+				error('Faust multiplication to a sparse matrix isn''t supported.')
+			elseif(isa(A,'matfaust.Faust'))
 				if (F.isReal)
 					C = matfaust.Faust(F, mexFaustReal('mul_faust', F.matrix.objectHandle, A.matrix.objectHandle));
 				else