This commit does two largely-unrelated things, but they hit the same code.

First, I tweaked the error messages a bit, to give better errors
for impredicative polymorphism.  This added the mb_fun argument to
tc_sub.

Second, I fixed a long-standing bug in tc_sub.  In the isBoxyTyVar case 
of tc_sub (rule F2) I was not recursing to tc_sub as the rule suggests,
but rather calling u_tys.  This is plain wrong, because the first
arugment might have more foralls.   

The solution is to recurse to tc_sub, but that in turn requires a parameter,
exp_ib, which says when we are inside a box.

Test is tc210.