OccBin: provide more informative error messages if name tags are not identical

If the name-tags are not identical as in

[name = 'borrowing', bind='borrcon']
lb = 0;
[name = 'borrowing1', relax='borrcon']
b = M*y;  

the error message will be

ERROR: the number of equations marked [static] must be equal to the number of equations marked [dynamic]

borrcon.mod

We need to make sure that the tags come in pairs and that there is no other equation that already uses that name (as to have three equations with the tag).