Predefined error value for unexpected number of safe bags

[piusalfred]

I have been working on a project where this error is significant

go-pkcs12/pkcs12.go

Line 256 in 426fe5a

err = errors.New("pkcs12: expected exactly two safe bags in the PFX PDU")

Can we create a predefined error value for this?. It makes it easy for error value comparison. I suggest something like this

var ErrExpectedTwoSafeBags error = errors.New("pkcs12: expected exactly two safe bags in the PFX PDU")

So We can do something like this

pfx, cert, err := pkcs12.Decode(pfxData, password)
if errors.Is(err,pkcs12.ErrExpectedTwoSafeBags){
    // LoadChain
}

instead of

if err.Error() == "pkcs12: expected exactly two safe bags in the PFX PDU" {
}