better error message for byte field of unknown size

laymanshex.go

@@ -16,7 +16,11 @@ import (
 
 func optPanic(msg string, err error) {
 	if err != nil {
-		panic(errors.New(msg + ": " + err.Error()))
+		if msg != "" {
+			panic(msg + ": " + err.Error())
+		} else {
+			panic(err)
+		}
 	}
 }
 
@@ -174,7 +178,11 @@ func (part *filePart) setByteFieldHandling(assignments map[string]*formatAssignm
 			ass.used = true
 			err = nil
 		} else {
-			err = errors.New("Could neither parse size to int64 nor obtain value from -fvar")
+			msg := "byte field: could neither parse size to int64 nor obtain value from -fvar"
+			if part.name != "" {
+				msg += " definition " + part.name
+			}
+			err = errors.New(msg)
 		}
 	}
 	optPanic(part.name, err)