error should be the last type when returning multiple items

[siddontang]

as golint warns: error should be the last type when returning multiple items
we should return the error as the last item, but in hookfs, nearly all the Interface functions break this rule.

I think we need to adjust them but this will break the compatibility. But I think it is worthwhile.

/cc @AkihiroSuda

[AkihiroSuda]

+1, I think breaking compatibility is ok, because vendoring has been very popular in the Go world.

[siddontang]

ok, I will improve it later.