This are handy utilities to automatically prepend context on error
return. For example in

	func myfunc(...) (..., err error) {
		defer xerr.Context(&err, "error context")
		...
		if ... {
			return ..., errors.New("an error")
		}
		return ..., nil
	}

while preserving nil error return on successful execution, myfunc will
return error with string "error context: an error" on failure case.