Spec changes.

* All errors during state transition result in an invalid tx

core/error.go

@@ -146,3 +146,19 @@ func IsKnownBlockErr(e error) bool {
 	_, ok := e.(*KnownBlockError)
 	return ok
 }
+
+type ValueTransferError struct {
+	message string
+}
+
+func ValueTransferErr(str string, v ...interface{}) *ValueTransferError {
+	return &ValueTransferError{fmt.Sprintf(str, v...)}
+}
+
+func (self *ValueTransferError) Error() string {
+	return self.message
+}
+func IsValueTransferErr(e error) bool {
+	_, ok := e.(*ValueTransferError)
+	return ok
+}