x/tools/go/types: misses '-' in error message

On the following program:

```
package a
var x = -byte(1)
```

`gotype` says:

```
tmp.go:2:10: byte(1) (constant -1 of type byte) overflows byte
```

It should say `-byte(1)`. `byte(1)` does not overflow byte.

on commit e9a746de6e61b9b823a981843c0d98bc209eed2b
