I must admit that `%v` has different output than I expected: http://play.golang.org/p/y5C3BRRyAc
This is clearly not the code part where we need (or should) optimize the assembly, but here is the assembly generated by 6g from tip, anyways: http://pastebin.com/bgn6PSsM
`"%s", err.Error()` results in slightly different assembly.