added msmbuilder.error.baseclasses.MSMError back. It currently has two methods: get_eigenvalue_variances and get_eigenvector_variances. I could also add a get_timeseries_variances or get_observable_variances which would calculate the variance in an observable's timeseries and the ensemble average value of an observable. These could actually be written in the base class since they would use the get_eigen[value,vector]_variances methods (I think). I haven't worked out the math yet...