For every sort of measurement, or constraint derived from data, you have uncertainties related to random noise (either related to do measurement error, or sampl

For every sort of measurement, or constraint derived from data, you have uncertainties related to random noise (either related to do measurement error, or sampling error caused by finite number of objects in your sample). But you also have systematic errors, for example, if your measurements are biased in some way. For supernovae, we know that the galaxies in which they explode are different in different redshifts, so it is possible there are effects that make supernovae brighter or fainter in different redshifts that have nothing to do with dark energy, but instead are related to chemical composition of stars. This is an example of systematic effect that can affect any cosmological measurement using supernovae. People try to control for those, but at the moment the uncertainty on cosmological parameters from SN is dominated not by random component of the error (i.e. related to number of SN), but the systematics and our ability to constrain them.