Change disp() to errors in var_sample_moments.m

Otherwise, the data moments that are required for estimating DSGE-VARs are set to an empty matrix instead of aborting. This will then lead to a cryptic crash in initial_estimation_checks.m due to matrix dimensions not agreeing.
Merge pull request #615 from JohannesPfeifer/dsge_var

Change disp() to errors in var_sample_moments.m