Provide more informative error messages for problems in model-simplification

The attached file has a variable defined as 0, which causes the preprocessor to replace it by 0. But this results in a cryptic error message

ERROR: Division by zero!
terminate called after throwing an instance of 'DataTree::DivisionByZeroException'

There is no clue for a user how to fix it. GT.mod

Similarly

sim1_linear_fail.mod

returns

Error: In not used in the model block

because the variable is always multiplied by 0.