suggest cmpopts.EquateErrors in error comparison panic

[neild]

This panics:

cmp.Equal(errors.New("x"), errors.New("x"))

panic: cannot handle unexported field at {*errors.errorString}.s:
	"errors".errorString
consider using a custom Comparer; if you control the implementation of type, you can also consider using an Exporter, AllowUnexported, or cmpopts.IgnoreUnexported

The panic could detect when the type being compared implements error and suggest using cmpopts.EquateErrors.