Update xdr; handle marshalling errors

Godeps/_workspace/src/github.com/calmh/xdr/reader.go

@@ -5,14 +5,12 @@
 package xdr
 
 import (
-	"errors"
+	"fmt"
 	"io"
 	"reflect"
 	"unsafe"
 )
 
-var ErrElementSizeExceeded = errors.New("element size exceeded")
-
 type Reader struct {
 	r   io.Reader
 	err error
@@ -71,7 +69,7 @@ func (r *Reader) ReadBytesMaxInto(max int, dst []byte) []byte {
 		return nil
 	}
 	if max > 0 && l > max {
-		r.err = ErrElementSizeExceeded
+		r.err = ElementSizeExceeded("bytes field", l, max)
 		return nil
 	}
 
@@ -162,3 +160,7 @@ func (r *Reader) Error() error {
 	}
 	return XDRError{"read", r.err}
 }
+
+func ElementSizeExceeded(field string, size, limit int) error {
+	return fmt.Errorf("%s exceeds size limit; %d > %d", field, size, limit)
+}