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.

Author: pnkfelix

src/librustc/middle/check_const.rs

@@ -307,8 +307,9 @@ impl<'a, 'tcx, 'v> Visitor<'v> for CheckCrateVisitor<'a, 'tcx> {
                             match const_eval::eval_const_expr_partial(self.tcx, ex, None) {
                                 Ok(_) => {}
                                 Err(msg) => {
-                                    span_err!(self.tcx.sess, ex.span, E0020,
-                                              "{} in a constant expression", msg)
+                                    span_err!(self.tcx.sess, msg.span, E0020,
+                                              "{} in a constant expression",
+                                              msg.description())
                                 }
                             }
                         }