Add detail for required field error

[PePoDev]

Description

I use go-env module in our project, It's was cool.

But I found ErrMissingRequiredValue does't show important detail about required field that miss.

Then I try to add what's required field that missing in error message 😊

[hinshun]

Prefer if you just refactor to:

type ErrMissingRequiredValue struct {
    Value string
}

[PePoDev]

@hinshun I'm not sure if that's right.

[hinshun]

Thanks @PePoDev !