Address arith-overflow and error-handling in const_eval.rs.

1. Detect and report arithmetic overflow during const-expr eval.

 2. Instead `eval_const_expr_partial` returning `Err(String)`, it now
    has a dedicated enum of different cases. The main benefit of this
    is the ability to pass along an interpretable payload, namely the
    two inputs that caused an overlfow.

I attempted to minimize fallout to error output in tests, but some was
unavoidable. Those changes are in a follow-on commit.

src/test/compile-fail/eval-enum.rs

@@ -9,8 +9,8 @@
 // except according to those terms.
 
 enum test {
-    div_zero = 1/0, //~ERROR expected constant: attempted to divide by zero
-    rem_zero = 1%0  //~ERROR expected constant: attempted remainder with a divisor of zero
+    div_zero = 1/0, //~ERROR constant evaluation error: attempted to divide by zero
+    rem_zero = 1%0  //~ERROR constant evaluation error: attempted remainder with a divisor of zero
 }
 
 fn main() {}