Add global CtonResult and CtonError types.

These are for reporting the overall result of compiling a function.

lib/cretonne/src/verifier/mod.rs

@@ -59,6 +59,7 @@ use ir::instructions::{InstructionFormat, BranchInfo, ResolvedConstraint, CallIn
 use ir::{types, Function, ValueDef, Ebb, Inst, SigRef, FuncRef, ValueList, JumpTable, StackSlot,
          Value, Type};
 use Context;
+use std::error as std_error;
 use std::fmt::{self, Display, Formatter};
 use std::result;
 use std::collections::BTreeSet;
@@ -78,6 +79,12 @@ impl Display for Error {
     }
 }
 
+impl std_error::Error for Error {
+    fn description(&self) -> &str {
+        &self.message
+    }
+}
+
 /// Verifier result.
 pub type Result<T> = result::Result<T, Error>;